Great responses above.
As a practical matter, you will probably (hopefully!) not have the occasion to use your sausages in Coz. unless you want to use them. Assuming all goes well, your DM will likely send up a sausage for the group at the end of your dive.
That said, you definitely should both have sausages in case you get separated from your DM.
If you're just getting started with diving, I would recommend that you not practice deploying the sausage while at depth just yet. There are a lot of things that can go wrong while doing this. If you get separated from your group in Coz, I'd suggest ascending slowly and carefully, doing a 360 degree scan for boat traffic before slowly breaking the surface with your arm first then the rest of your body. Once you're on the surface and positively buoyant, you can then inflate your sausage to help the boat find you. This is what I'd suggest practicing.
--Scott