for my 5th starter spot, I tried to set up a "tandem" by setting tandem 1A and tandem 1B, thinking that the sim will bring in one after another in the game.  However, when trying to save, it says that Tandem Starters must be the last spots in a pitching rotation.

I don't understand.  Please help.

Don't try to make sense of it.  It's just a limitation of the game.

You're correct about how tandems work.

You can have up to 4 tandem starter pairs.  If you have less than 4, you have to number them from the bottom up.  First one must be Tandem 4.  Next Tandem 3.

Other than the 1st game of the season or playoff series, it's easy to work around.  Just renumber your starters so the tandem you want is SP 4 if you have one, SP 3 if you have two, etc.

Marking pitchers as Tandem 1A and 1B means they will jointly occupy the first spot in your rotation, not the last.

The problem is that if you use a tandem/starter combination, you can only have 3 starters followed by your tandem set (SP1, SP2, SP3 and T4A/B). Given that a Tandem 5A/B does not exist, it is impossible to set a 4 plus Tandem rotation except by rotating a pitcher in and out of your rotation every few games (highly annoying). Therefore, unless you want to deal with the headache of moving pitchers in and out, tandem simply isnt worth the bother. WIS has a few times said they would add Tandem 5A/B, but never came through.