Parallel wiring causing oscillation

[MmmPedals]

Just wired up 2 overdrives with a series parallel switch. It is wired so that in parallel mode with only one OD on it is OD and clean signal in parallel.
Soooo... with one OD on the signal makes its way back to the input and around we go.
Any easy fixes?

[edvard]

Obviously these are true-bypass?
In that case, you need some sort of mixer circuit at the end instead of simply paralleling.
That will isolate the outputs from inputs else you get feedback as you found.

How to integrate that into a series/parallel circuit I'm not so sure.