Personally, I wouldn't be mixing incremental and differential, I can't see any mix of these being a best practice.
Does your Sunday backup fit on one tape? It so, the choice is simple. Replace the lot with two jobs, one for each server. Then have two jobs (one for each server).
Otherwise, pick a system (incremental or differential) and stick with it. Then have four backup jobs (each server, different jobs).
I do find per server jobs easier to manage, but then, it depends how many servers you have. Anything that results in more than 6 jobs or so becomes annoying to schedule, so in that case, I make one job with multiple servers.