You're not the first to ask about this, and you won't be the last.
To remove these ambiguities, the answer is very simple:
- Co-term all certification cycles for multi-cert members.
- Prorate any first cycle CPE requirements for any new certifications for multi-cert members.
Why this hasn't been implemented already, I have no idea. I have raised the very same points on numerous occasions in supposedly official channels designed to elicit feedback from the membership to no avail.
Right now, in terms of your forthcoming CCSP, if you pass and are endorsed, you will have a brand new, individual CCSP certification 3-year cycle for which you'd need to submit 90 CPEs. Every CPE you've submitted for your CISSP already will have no bearing on that.
Once you have been formally awarded the CCSP, when you earn CPEs, it's likely they can be applied to both your individual CISSP and CCSP maintenance requirements.
The same is true of any CISSP Concentrations you go on to earn, other than to say these are already co-termed to your CISSP certification in terms of cycle start and end dates. Additionally, there has already been some guidance issued around mid CISSP cycle CPE requirements for Concentrations:
In summary, outside of being awarded a Concentration in the last six months of your CISSP cycle, you'd still be required to submit the required 20 CPEs.
e.g. Get awarded a Concentration one day into your CISSP 3-year cycle, you get the best part of 3 years to submit the 20 CPEs. Get awarded a Concentration six months and one day before your CISSP 3-year cycle ends, you still need to submit the 20 CPEs. Get awarded a Concentration six months before your CISSP 3-year cycle ends (i.e. just one day less than the previous example), you don't have to submit any CPEs during the current cycle.
Makes sense? 🙄