I'm sorry but I don't think I understand what the problem is- could you maybe provide more detail?
It's of course possible for cats to go into heat at the same time- are you just wondering if it's possible or is it problematic? Also, if you have two girls in heat make sure you are taking extra precautions that they don't have any access to the outdoors or to any intact males. Otherwise you are going to be dealing with simultaneous pregnancies!
Please provide some more details so that we can better advise you. Welcome to TCS!
To my knowledge, loose stools are not associated with a cat going into heat so there is likely some other cause. Was it just a one time bout for each of them or has it been ongoing? Many things can cause diarrhea and if it's just the occasional instance then it's not really a cause for concern. However, if it's lasting for more than a day then you really need to have them evaluated by a vet to be sure there isn't something more serious going on. Especially since it's happening to both of them I would be concerned about something like an infection or parasites.
So, I would say err on the side of caution and have them both checked out by your vet. That would also be a good time to discuss having them both spayed.
Vibes that there isn't something serious going on and that they are both healthy.