There is a rule:
"If more than one club from the same city, or within a radius of 50km (31 miles) of each other, is taking part in the competition 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, the UEFA administration may alter or confirm dates and kick-off times according to the principles set by the Club Competitions Committee."
I understood the distance to be just under 40 miles, but as the crow flies it may be shorter. This site says it's 45 km:
http://www.sportmapworld.com/distance/lyon/saint-etienne/
Either way, it would have been given to UEFA pre-tournament and not in the last month (as far as I can tell).