@RDub, please correct me if my assumptions are wrong. You are asking (I think) if you need to worry about damaging one HT when you transmit on the other in a "normal" way. You're not talking about an HT that you've connected to an external antenna, right?
If that's the case then, yeah, you should think about it but it's not necessary to worry much.
There's a few gotchas in all of this - the amount of power, the antenna gain, bands you're using, proximity and orientation. The worst case is lots of power into antennas close and parallel to each other on the same band, which is what I'm talking about with 50 watt mobile radio(s) into this situation. This is a definition of "Do as I say, not as I do." if ever there was one.
With HTs the power is 10 dB lower (5 watts is +37 dBm for calculations) and the antennas usually have around 2 to 3 dB less gain. Then moving them so they aren't so close and parallel makes a major difference in coupling. If you're using a GMRS HT and a VHF/2m monoband HT then you have out of band filtering on each that helps.
Bottom line is you mostly don't need to be concerned.
The one situation I might caution is if you're using a speaker/mic with your radio(s) and have them both sitting next to each other in a cup holder or something. That would be potentially a tough situation to do a lot.