ASAMI IN DEFENSIVE MODE FOREVER ABOUT MAKO BEING ALONE WITH GIRLS LOL

They gotta tag team at some point, but Mako is essentially the only real 'weak point' that Asami will have later on, and if she starts slipping on the desire to work with them they'll kidnap his ass xD. TenLin totes gotta work, and your idea is good, maybe Asami does accidentally expose them to something. If Xana and Xano can basically bottle bending and harness its powers to simply hand it out to those they deem worthy of it, it would not surprise me if the twins were working on a way to maintain youth and also unlock the key to immortality.