i dont think it works that way, alblack.
the way substitutions work, when there are multiple legal positioning for the 5 players who should go in, is it minimizes the sum of their positions on the depth chart. you can't control the tie breaker. for example -
if you have the backup C at the 2nd C spot, and 3rd PF spot, and the backup SF at the 2PF spot and 3C spot - when they are both in - you will ALWAYS see the C at C, the SF at PF. in that case, the sum of their spots on the depth chart is 4. if the C was at PF, and the SF at C, the sum of their spots on the depth chart would be 6.
in your case, the C is 2 for PF/C and the SF is 3 for PF/C. when you need both to play, with C at C, or C at PF, the sum of their spots on the depth chart is always 5. that is why you are seeing what you are seeing.
to get around this, you should move the SF to the 4th spot on the C list, if you can afford to do so, or move the C to the 4th spot on the PF list, if you can afford to do so. you should probably be able to do one of those two (either will work). if you have a big who can play backup, behind those guys, you'd ideally want him at 4th at both, but you just have to decide what is more important, i guess. ive asked (and other coaches have too) for the depth chart to go to 5, that seems like it would give enough control, where in the current setup, there are times you have to make a tradeoff, that you probably shouldnt have to make.
hope that helps!