Okay, I'm trying to draw the picture here… Alright, here it goes:

Regular combat: Sans has a very low chance of doing anything (besides buffing/cheering Papyrus), while keeping his trademark dodge (dodging would prevent whatever action Sans was set to perform, though).

If Papyrus is knocked out: Carrying Papyrus on his back (just like the Mario Bros.), he would be unable to dodge: You would need to actively act, but in exchange, Gaster Blaster is unlocked as well as doing nothing to bail out or spend it to ressurect Papyrus (this would give, say, a brief time where Papyrus is alive but Sans can't auto-dodge, and his stance would change from tip-toeing to cautious). Dying as Sans during this "vulnerability" while attempting to dodge would turn Papyrus into AU Papyrus (just idling to "exploit" this would let Papyrus acknowledge that his laziness got to him).

To deal with AI either avoiding Sans entirely or blindly attack both (effectively giving Papyrus 50% dodge), a "swap places/rotate – can't jump that turn" feature could be added.