I've always seen as learning and playing the guitar as something fun and relaxing, and it had always been something I wanted to do. As long as it was on my terms.

I was forced to learn an instrument, and I was made to play in the school band. I refused to practice outside of school, I refused to learn how to read sheet music, I refused to learn music theory, and I actively attempted to ruin any performance I was forced to be a part of. Random notes as loud as I could play them throughout the entire performance, to include what was supposed to be a violin solo. I'm fairly impressed with the level of calm and patience from the instructor/band leader/conductor. I was informed that I did not have to remain in the school band if I did not want to. I had made it very clear from the first day that learning the instrument was not something I had wanted to do, and that the school band was not something I wanted to be a part of.
There literally weren't any upsides to having been forced into playing.

I'd say forcing your children to do something that should normally be fun is a generally bad idea. As soon as practice feels like punishment, then you've lost whatever long term benefits there might have been.

Of course, I was a rather stubborn and independent child, but yours might be weak willed and lack character. If so, give it a shot and see what happens. Or hedge your bets and make one of them learn. You could even be clever about it and force the other to do brutal cardio exercises until they pass out from exhaustion, then after a few weeks ask them (separately) if they want to switch. It'll provide you 3/4 odds that at least one of them will choose the guitar, and 1/4 odds that you'll have two track stars with ever mounting therapy bills. Of course, they're your kids, so you get to pick how to raise them.