How to introduce crabs?

Is it safe to introduce a crab into my tank if my other crab is down in the substrate? She’s been alone for almost a year and I’d love to get a new crab eventually but I’m scared to introduce a new one. She’s been under the substrate for like 2 weeks now (I can’t tell if she’s molting because I can still hear her moving around a bunch) but when she does molt she usually takes a few months since she’s a larger crab. Is it best I wait till she comes back up? What’s the best way to introduce crabs?