Hi Anon
I'd use "was a realist" in your first sentence, and in your second sentence I'd use "were qualified".
I think I'd follow the same sort of rule we use for "or" in this case: the use of the singular or plural form of the verb is determined by the noun closest to the verb.
What your sentences mean are:
- Kosaka was a realist, and so was Sakamoto.
- The trainer was qualified, and so were the trainees.