Actually you can just connect ZF7731 to "PoE out" port of ZF7762, than ZF7731 will be powered from it, and you need just one PoE injector to power ZF7762. It is designed exactly for such use, and included with ZF7762 PoE injector is 60W.
The only drawback is that this second port is 100MB/s, not 1GB/s, but it will be enough in most cases.
You can also power both APs from 12V DC or use 2x PoE injectors, if you wish.