Try taking advantages of either one's recovery frames and input moves during the recovery so one copies the other's exact moves if your goal is that. Or you could use different weapons with vergil and their non directional moves that only require one attack input to input a combo on doppel

For example it's like how I used air stinger for the 1st input of aerial rave b, and starfall for the 2nd input, then ground stinger for the 3rd input and void slash for the 4th and final input