Q: Can Apple Pay & Google Wallet cohabitate?

For what reason? Apple Pay processes payments and Google Wallet processes payments.  Why does one-single-person need two 'keys' to the same lock?

As far as I know, Google Wallet on an iPhone will require you to open an app. Apple Pay can be initiated directly from the lock screen with Touch ID.

However, to answer your question, you'll just need to select Google Wallet if that's your preference before making the payment.