How do I connect AVR & amp into same sub? Please help?

Hi there. I
was wondering if someone could help me with some connection concerns. I
currently have my 2ch amp connected into a sub for music but have just
purchased an AVR for movies which I also want to connect into the same sub. The
amp has L and R sw cables going into the sub’s phono inputs. Can I connect
another sw cable from the AVR into the sub’s left phono input via a 2into 1
adapter? Or, will this be disastrous? Only one of either the 2ch amp or AVR
will be used at the same time. Thanks for your help.