What is your thinking on how you'd control speed and direction on the conventional engine? Does the conventional engine have a reversing-unit of some type?
Assuming you could reduce the voltage with diodes, how are you going to prevent jackrabbit starts. Personally I wouldn't run that way but if you're up to trying, go for it and please let us know the results.
Now, on the other hand, if you're thinking about running a conventional electric like a GG1 no problem other than installing a catenary system. I've been toying with this idea for some time - two separate locomotives, two separate voltage sources and one common track.