Kalista-Sion ult interaction

Earlier today I played a game as Kalista with my friend being Sion support. We were in a fight and I suddenly got surrounded by a bunch of enemies in melee range, so I used my ult. At the same time as I used my ult, Sion did the same, also in an attempt to save me. Both of us using our ult at the same time caused him to be unable to do anything until my ult(Kalista's) wore off, afterwards he was still unable to do anything as if still ulting(how he described it) for a few seconds.