First, you need to split up the unit and have two separate reservations as a result. Then, you can get guest certificates for each side.
The lockoff functionality is not well implemented at Hyatt. If you are within HRPP, you only have three choices:
1. Reserve the whole unit
2. Reserve the large side
3. Reserve the small side
You cannot reserve both sides separately when in HRPP, at least online. Not sure, maybe it's possible to do by phone.
If you already have a single HRPP reservation for the whole unit, you cannot lock it off online either, to end up with tho separate HRPP reservations for both sides. Again, not sure, maybe it's possible to do by phone. But I remember reading someone was unsuccessful trying to achieve this exact outcome.