I've never understood what it means with Tandem #1A and so on for pitchers. Can someone explain this, I know it's used for spot starts in the playoffs.. I think, but don't know the first clue on how to use it.

You set a tandem rotation and tandem 1A pitches until he reaches his pitch count or is pulled due to ineffectiveness, at which point tandem 1B would come in and do the same.

hmm now how would you go about doing a spot start with a pitcher?

Just manually move him into the rotation.

and remember, for reasons known only to WIS and the shadow, you must keep your tandems in the last spot(s) in the order  when you make the move.

It's not necessarily just for spot starts or the playoffs. If you want, you can use tandem starters all season long. I've never done it myself, but I think it much be a good arrangement if you have a couple of really good pitchers who aren't durable enough to be starters. That way you can ensure they pitch on a regular basis, but they don't have to pitch a regular starter workload.