Having 2 different preamps to turn on the same amp via 12v trigger

I have a an av pre for home theater and a 2 channel BHK pre for audio. The BHK is used in bypass while using home theater. 
My issue is they share the same amp and I’d like for the amp to trigger on with either pre. The amp doesn’t have line input sensing trigger. 
How can I trigger the amp with either pre ?  
So far I’ve only come up with make my own cable with diodes as to not backfeed to the pre that’s not in use. Wish I can find something off the shelf.