@FoodieMcFoodieBigBack I must say, stressful or not, this is a nice position to be in so congratulations on the AC's!
As Firm A had more options available, I would first reach out to them to ask if you could change the date. If there is no leeway there, then try the same with Firm B. If however, both are quite stern, then you're going to have to stick with the firm you prefer the most. However, that doesn't mean to say the other is cancelled per say, you should ask if you could be considered for a future scheme and see if they would carry your application over for AC for that one.
Good luck! I hope either firm can honour the date change!