I'd guess it would be weaker then just one or the other if you selected the moves randomly. Different engines have strengths in different areas, or so I've heard.
The reason human+computer is stronger than either alone is because the strengths of one cover the weaknesses of the other. Computers can play more or less perfectly in the short term while humans can think abstractly and plan indefinite moves ahead without using calculation. So ideally you get the best tactics and strategy.
In centaur chess, humans and computers aid together forming a killer combo. So I thought - how about combining two (or more) chess engines?
I thought of randomizing their moves. Say, if we want to combine Stockfish and Houdini. Then in each move, we flip a coin. If heads, we let Stockfish play. Else, make Houdini to move. Or, if we know the strengths of each engine, we could make one move at a higher probability than the other given that particular situation.
When played like this - some moves are going to be strong or weak according to the engines. But then, they are the strongest moves for them when they move!
So, I wonder if the resulting combination would be stronger than the two. Any ideas on this?