Will my co-member be able to access my reservations as well?

Yes and no . . .

YES for vehicles available by reservation

Your co-member will be able to access the vehicles that you have reserved in your name without any problem using their own RFID key fob (or equivalent) they usually use, and vice versa. You must use the same RFID key (or equivalent) to start and end a trip.

NO for FLEX

The above is true in the case of station-based vehicles available by reservation, but not in the case of FLEX: if you block a FLEX vehicle, only the person who blocked the vehicle will be able to access it (unless you lend them your RFID key or equivalent).

No with the mobile app

Any reservations made on your account will not be visible to your co-member in the app. Therefore, it will be impossible for them to start a trio with the app unless they create the booking on their own account.