I'm just theorizing here, but the issue here is to remember that YOU don't have that DKP. Your CHARACTERS do. If one single-boxed character cannot transfer DKP to another single-boxed character, then your multiboxed characters should NOT be able to do so either. Basically, the rules should apply to each of your characters like they do to everyone else's. If everyone has that understanding up front, it should be fine.

This goes to the heart of why the ignorant and naive in the WoW community think multiboxing gives an advantage - they tie their in-game gains to their personal gains, which is ridiculous.