Do the PoE injectors supply power on both directions? I may need to do a back-to-back AP scenario and would prefer to power both APs from 1 injector/power supply.

Normaly not.

connect then both to an omnitik .... then you also have a backup device ...
or use a multiport poe injector

jou can also use 1 POE injector with cable (male - female )+ 1 poe injector with 2 female plugs.
you need to have 2 DC plugs, and jou can connect them to the same powersupply.

Do the PoE injectors supply power on both directions? I may need to do a back-to-back AP scenario and would prefer to power both APs from 1 injector/power supply.

You could use an RB750UP, which has 4 POE out ports.http://routerboard.com/RB750UP

Yeah, I think we'll use the OmniTIKs with the PoE out option if we have to go this route.