I have never used tandems before but i took over a team with very thin SP but very good RP with high sta/dura. 

I'd like to position 4 normal starters and use 2 relievers in tandem in the 5th spot.  But i cant figure out how to do this.  There is no 5a/5b and no matter what I do i get the error message "tandem starters must be the last spots in a pitching rotation.

???

For whatever reason you can't use a SP5 with a tandem. I ran into this once and put the 5th guys in rest when not starting them. Kind of a pain, requires a shuffle to set them 4A & B and then put them back in rest and re-set a starter, but since I check almost every day anyway wasn't so bad. I can't know the reason "they" don't allow tandem 5A & B instead of SP 5 & 6, seems lame to me. 

Yup - you can't do a tandem 5.  I've on occasion managed to run a rotation with 1 or 2 really durable starters working as S1 & S2 and then using 4 guys to split the last two spots.  This lets you get away with some lower stamina/durability guys in the tandem roles, but you need a real horse or two at the top.