I seem to remember that a second head designer can be hired and assigned to some parts? I might investigate that as an option, should help spread the workload...
Im not sure it is the case though as i have 3 head mechanics in my squad and the active head has Very Good moral and motivation while to two inactive ones have Superb for both stats, weird!
This is very interesting though.. I had no idea you could have multiple designers and have different ones working on separate projects. I would imagine that the result would be better than say one poor haggered individual managing 5 or 6 projects.