There is a 50km rule but only if applied by the association or clubs when entering. I would expect that neither the SFA nor ourselves would have objected to Kilmarnock playing at the same time. Rule quoted below with my bold.
23.03 If more than one club from the same city, or within a radius of 50km (31 miles) of each other, is taking part in any of the UEFA club competitions and/or plays in the same stadium, and if the association and the clubs concerned explicitly declare when entering the clubs that their matches cannot be played on the same day or on consecutive days, the UEFA administration may alter or confirm dates and kickoff times in accordance with the principles set by the Club Competitions Committee.