LED to get power from two transistors

I have a single led in the circuit. I plan to power it from two transistors one of which or both could get the signal to turn it on.

Should I use a diode to prevent backfeed?

It depends entirely how you connect it - the entire point of 'open-collector' outputs is that you can simply connect them together and they work as an OR gate.

So simply connect them as open-collector.