"Thanks for the offer. I'm still lookin' for a situation that feels right to me. Good luck with your project."
On the other hand...
I had banked on the idea that one of them wouldn't want me to join their respective band.
First, if you didn't want to jam with them (or anyone else) in the first place, I think you should have turned them down upfront.
So now you kinda wasted their time and yours "
banking" on something they would do that you couldn't and now you want to flake out on them?
I suggest you learn to be more assertive in stating what you want/need (which is under your control) upfront instead of depending on the actions of others (which is not under your control). Learn to deal with confrontation (or what you perceive as confrontation) head on. Take a stand and stand by it. No need to be mean or yell and scream to get what you want. No need to seek other's approval before you do or not do something. Sometimes there will be consequences you may or may not like but why not chose your own way?
Don't worry about "hurting" anyone's feeling's when you speak up. At least they will respect you for making a decision rather than not.
I mean, before you went out to jam with them you could have let them know that you wouldn't be able to make any commitments. Or just said "no thank you" when they asked you to jam.
Also, jamming with someone, IMO, is not really "
joining a band. I you want to jam but not make a band, see if they would be interested in doing that on a semi-regular basis with no commitments, if that interests you.