2 macs with small ssds. One backup drive or multiple

So I have a new Mac Pro with a small 256gb ssd hard drive. I have a larger 2tb drive that i run backups.

My girlfriend also has a macbook air with a tiny 128 gb ssd.

Our problem is our iphones have 50gb of music / pictures / apps so backing them up on itunes on our boot drive would be suicide. right now I have an old mac pro with plenty of disk space i use for my iphone but i want to use my primary computer.

Whats the best way to have everything with iTunes/iPhoto just goto an external drive? And is it possible for us to both use the same set of drives or no? Could we just use an apple Time Capsule for both our machines? And then couldnt We just figure out how to make itunes and Iphoto look for that network drive to work?