Are you sure they are both HOs and not a HO and a SHO with the same stats?
Yes I bid on and bought a stack of 4 on the market planning to slot two and "+" both of them.
Edit: I'll see if I can get some screenshots. I don't know if it's possible to capture the enhancement screen or not.
Ok I found the problem. I was having a derpy moment. Now that I'm level 48 it worked fine. 
I tried to combine some level 50 HOs recently to "+" them and I got a message something like "Are you sure you want to replace this enhancement with one of lower level?" Both were level 50. I tried using one in my tray (dragging it on top of one already slotted) and also tried combining two identical ones already slotted into the power, but the game won't let me combine them.
So has there been some change to combining HOs that I missed? I try to keep up with patch notes and I don't remember seeing any change to that. I know I used to do it in the past all the time, so I don't know what's changed. Maybe I'm missing something simple?