Yes, two cameras with two transmitters is possible, but I'm not sure how well it will work out in practice. A couple of things:
- Do you really want a GoPro for the piloting feed? You won't get all that resolution sent to your piloting monitor. You can buy REALLY inexpensive and REALLY light ccd cameras, and I think I've even seen some combo kits with a camera and transmitter all mounted together in a small package that you could tape to the top of your Phantom.
- From what I understand, there is quite a bit of "bleeding" into other channels with the video transmitters. That is, if your piloting transmitter is on Ch 1 your filming receiver on Ch 2 or 3 might pick up Ch 1 too. You should pick channels as far apart as possible. Or, you could use an entirely different band for the piloting feed.
In my opinion, it would be simpler to use a setup that allowed video input switching. The DJI iOSD Mark II does this. It takes two inputs and allows you to switch between them. I would still use a cheap/light camera for the piloting feed. If you're going to go the two transmitter route, you really should be comfortable with the various parts of an FPV system. There are some great videos in the FAQ section of fpvlabs.com