In a higher salary cap league, I plan on using 2 steroid era stud starting pitchers as a tandem a/b with 2 deadball era high IP starters. My rotation would be 1,2,3a/3b. The steroid era guys would then start every 6th game. would I then be able to pump their pitch count up to 110 or higher? Or would the in game fatigue kill me?

First of all, that's not how a tandem works at all.  They don't start every 6th game, they each pitch (back-to-back) in every 3rd game.  Most experienced users including myself agree that tandems are generally going to win you more games.  However, if you want each to start every 6th game just use starter #3A and starter #3B - NOT tandem.  Secondly, it entirely depends on the guys.  2000 Pedro has about 7 1/2 IP/G and also a high K rate.  He can easily go 110 pitches.  '94 Maddux can also easily go that far.  '95 Maddux would be a little fatigued by 110 but could probably do it.  Some of the top Kevin Browns might struggle past 85 or 90 pitches.  Again, though, if you put them in a tandem they'll be something like 50 or 55 target, 60 max and you won't have to worry about it at all.