Whether or not your daughters want to be on their tummies, it is truly the best thing for them. For some odd reason, they learn to breath deeper when on their tummies, and they help develop their lungs further by the screaming they do.
I was told that 5 times a day to put my son on his tummy for 5-10 min. each time. Don't give in to the screaming, at their age, it will help them learn to roll over, if they aren't doing so already.
Don't give up on it. Continue to have tummy time daily. Each week add a couple of minutes every time they are on their tummies. It is good for them to scream for a little while.
Some parents hate hearing the screaming and think it is hurting their child, but truly let them scream for 5 minutes at a time for 5 times a day won't hurt them. It will help them in so many ways that it is unthinkable not to have them on their tummies.
This is what I was told, but my son was 6 weeks old when we were told this, after a few days, he started screaming less and less, and then after about 2 weeks, he stopped screaming all together, and then learned to rollover, followed by scooting at 5 1/2 months, crawling at 6 months, and walking at 9 months. Only occasionally do I regret putting him on his tummy, knowing that by letting him get so comfy, he would learn to crawl earlier, but for the most part I don't regrett it. I think he would have learned earlier regardless of tummy time or not.
Try rolling a towel up for them. This lets them sit up a little lower than a boppy, and not flat on the floor either. (we put rubber bands around it to keep it from un rolling all the time). the other thing to try is get down and let them either see your face or have them facing each other so they can see each other, this may comfort one or both of them. Lay on your back with your arms out and lay them either on one arm each or both on one arm, get a non-breakable mirror and a couple of balls or other objects to hit on the mirror, babies love looking at their reflections, and the reflections of other objects.
These are just a few suggestions on what to try, use them and expand them as necessary. I hope this helps. It helped me, and a few of my friends as well.
Good Luck, and don't give up!