I do a mix of running and cycling, and haven't noticed any adverse impact on my riding.
I don't do long runs, a maximum of an hour at best, and normally do one run a week, and three or four rides. Sometimes I swap that around, but I find that as long as I consistently ride, i.e. I don't just run every week for a few weeks, then I see no impact on my riding fitness.
Cycling can be good on a 'rest' day in a running week plan. To do a 60/40 mix you might find it doesn't help either particularly, but your mind might enjoy it to do a mix (mine does).
I do a mix of running and cycling, and haven't noticed any adverse impact on my riding.
I don't do long runs, a maximum of an hour at best, and normally do one run a week, and three or four rides. Sometimes I swap that around, but I find that as long as I consistently ride, i.e. I don't just run every week for a few weeks, then I see no impact on my riding fitness.