HCA: Hermit Crab Association

I work at a pet store (small, local, and actually meets the minimum for all the species we sell. we are encouraged to deny sales if there is an animal welfare concern )
and the other day someone had an emergency rehoming of purple pincher crabs. The store has a few from similar cases right now, but they were MUCH larger. I had an empty 29 gallon tank, so I took the 4 crabs home. She told me their names are Hermie, Squeezy, Dolly Lama, and Teddy. Hermie and Teddy are in painted shells, but she really seemed to know her stuff and was very heartbroken to have to rehome them. It was an emergency.
Luckily enough I kind of on-again off-again obsession with the little guys, and have been stockpiling supplies for them over the years. I wasn't going to adopt some until I had a 75 gallon, but I still feel good about this decision.
I got them set up in my cornsnake's old baby bioactive terrarium.
After I got them set up the first night, Teddy immediately tried on a, at least I thought so, more fitting shell (his is painted, and QUITE large for him) But he didn't seem to like it, and I had to break up a fight as Teddy returned to his old empty shell that Hermie was taking an interest in. I turned Hermie around to face the climbing wall, and he calmed down. Lots of chirping, very dramatic. I've been rotating the shells in there the past couple days, but I haven't seen anyone really shopping around for shells since.

Humidity looks good! 80% when I go to bed and 65% when I wake up to mist again.

here's an imgur gallery with some pictures!
https://imgur.io/a/POYS0Jq

QUESTIONS:

I've got two heat mats on there, but the temperature is staying at 71° on the warm side, 67° om the cold side. I turned my home thermostat up to reach that high, but that's not sustainable for my budget. Does anyone have any tips and tricks for warming them on a budget?

I bought a glass lid, but how the heck does anyone attach that back plastic peice?! I've been trying for days and I've actually cut myself up more than once so far.

I have a few dependable critter keeping friends I think could possibly take 2 of them. I've asked around, but do you guys think it'd be okay to split them up?

HCA recommends 3 to 5 extra shells per crab - have a look at the 'shells' care sheet to be sure you are offering appropriate types of shells for the species of crabs you have.
What brand of heat mats do you have? Some can be safely insulated over top. But even if you don't have the type that can be insulated over, you can insulate around them, so the sides and whatever of the back without a heat mat can be insulated. You can also put a folded up blanket on the lid to keep heat in - assuming you get your lid to hold. Sorry I have no advice for that.
It would be fine to split them up, crabs do just fine in pairs or even singles.

They're both Zoomed Reptitherm under tank heaters, one is rated 10 gallon and the other 20.

The shells are turbo snail in good condition, i boiled and then rolled the inital ones in saltwater but haven't done that since, until today when I got more. They're mostly green and tapestry turbo shells. There are a few other types I don't recognize with slightly pointier shapes, I've left them in the mix for variety's sake. I think they might be murex, turban, nerite, and some yellow banded snail shells. I'd read elsewhere Purple pinchers sometimes like these, but let me know what you think. Would you count them toward the vaiable shell total if it was your tank?

I've been setting glass lid over the screen one with saran wrap at the back uncovered portion on one side. That has helped the heat a ton but I worry it'll melt. I am very worried about any potential fire hazards, I won't be home to catch them with my job hours and longer commute time. I checked on it this morning and the temperature was actually 74° now so maybe it wasn't as bad as I worried. I have one extra mini heat pad I will probably add to the side.

And that is good to know about splitting them up being okay, thank you! I would feel better caring for just the 2 so that they have more space and shells, but won't get bored. If none of my friends want to brave hermit crab husbandry, I will probably adopt them out thru my work or responsible online resources.

Thank you so much for the welcome and for everyone involved in such a great community and resource bank. I've been a lurker for quite a while, and sent many people here for information at my job. We have your basic care guide as a qr code on our hermit crab tank. You all are amazing.

Found an old heat mat and reptile light dome lying around the house! Woo! In the dome is a regular house light, attached to a timer. Temperature is now ~80° on the warm side, ~70° on the cool side. Humidity is 75-80% on the warmer, drier side. I only have one hygrometer so I put it on the side I knew would be worse.
I've ordered more pothos for plant coverage, greensand, and worm castings.
I have sphagnum moss, catappa leaves, millet, popcorn, Snout and Shell crab foods, Gargeer crab pellets, coconut oil, aaaand mealworms. Anything I'm missing for happy crabby meals?

I have the extra shells scattered around the enclosure, but it seems popular to keep them in a tupperware. What's the benefits fo that?

That sounds good for your heat and humidity! The Zoomed UTH can not be insulated over top, due to the fire hazard. For crabs, we go more by the physical dimensions of the UTH rather than how many gallons it heats (cuz the UTH is used quite differently for reptiles, under the tank and no substrate). Many people have one big one across the back of the tank, but I'm like you and I have two on the back and one on each side. My tank is in a room where the heat varies greatly between seasons, so it works for me to be able to plug/ unplug them as needed.
Many of us use or have used plastic wrap on the top of the tank - I didn't have a lid for my ten or twenty gallon tanks, all I had was plastic wrap taped over the top. It won't melt unless you are using heat lights. I'm not sure what a 'reptile dome' is LOL.

oh good! thank you that makes me feel better. I shouldve clarified reptile dome lamp.