Will my wallet be extended or replaced when I recover another wallet in Breadwallet? Will my current balance combine with the balance on the other wallet I want to recover?

Or do I need to empty the wallet on my phone before recovering the wallet that was on my old phone?
If you start or recover a wallet it is completely isolated from your old one, the balance will not be combined. You don't need to empty it first, but it will be awkward to keep swapping backwards and forwards between two wallets if you choose to do so (you'll need to re-sync each time).