That's an interesting idea, Peter. I assume you are refereeing to competitive vs. cooperative training. By saying the arts are two sides of the same thing are each incomplete on their own? I think a case could be made that no system really covers everything. How does that fit into practical skills testing?
Actually Judo and Aikido are really jujutsu separated by distance. Where I trained in Japan crossing over for a bit was actually encouraged.
Your question was about vetting your skills and my answer really was given just in that light. There is actually full on resistance randori in my style of Aikido so for me the competitive vs cooperative issue was really not the reason although for most aikido styles I think it could/should be.
One of the main reasons is that no matter where and how you train you will fit into a particular box and it takes real effort to break out of that. Often when we adjust our training we really are just moving to a different corner of that same box. Judo, both in distance and training style is not so different but different enough that you can test your Aikido skills without that generated delusion that affects us all. Consider it the outside examiner that we see for Post-graduate courses.