Always check the NCL Ts&Cs online before booking since policies change frequently.
https://www.ncl.com/about/terms-and-conditions/promotions#cruisefirst
If the link doesn't work, go to NCL website, look under Cruise Deals tab, then select CruiseFirst. At the bottom of the page, there is a link to NCLs Promotions Terms and Conditions document. The Ts&Cs page shows everything - CruiseFirst, Cruise Next, Latitudes etc.
At present you cannot combine CruiseNext and Cruise First. I don't believe this has ever been possible.
As julig22 posted, you can use two CruiseNext certificates for any cabin type booked six months out booked by 28Nov22. It is likely to revert to balconies only after that date.
Tip for everyone, keep a separate record of your CruiseNext/CruiseFirst certificate numbers, amounts, expiration date and which was used for a booking. Your booking document will not show which certificate was used, only that a CN or CF certificate was used. Particularly helpful if you (or NCL) cancels a cruise and restores a certificate. The restored certificate may have a new number and a new expiration date. Helpful if you have multiple certificates and want to confirm that it was restored.