Combi steam usually goes on top.

I don't have a stack, just a single built in steam oven with a range, but I would put the steam oven on top. Steam / hot air rises and it'd be less likely to interfere with the convection electronics (not that they do, I'm just imagining this as a greater possibility).

I have a Convection Micro and a CSO stacked with the steam on the bottom, didn't want to reach up over my shoulders to get stuff out of the steam oven. Plus on the Wolf CSO the door opens down, so it would be even harder, the micro swings the door to the side.

Phil