Platies do indeed eat their fry. Fry do not usually survive well in a community. Some female platies will delay dropping their fry if they are stressed out. Breeder boxes tend to stress them out. Some platies will refuse to drop fry inside breeder boxes of any sort. Having both females in there together is going to be even more stressful for them. I suggest you only put one female at a time in there. You can always buy a second breeder box.
Personally, I use breeding nets. I don't put the females in there though, I just scoop out fry after they have been dropped. I have had a couple of females drop dead inside the nets apparently from stress/complications, so I don't use them anymore except for fry I find. The link jetajockey posted seems cool though.
The breeding traps do not allow he fish to swim around and they need to swim around and exercise to get the whole process going. I do not suggest leaving a female in a breeding trap for longer than 2 days. I would let them out to swim a bit and then you can put them (or preferable just 1) back in if you choose to. Make sure the breeder box allows for water exchange with the tank, otherwise switch out the water manually so ammonia doesn't build up inside the box.