As mentioned a few days ago, I have both a large FFC with "Begin travel by 12/31/22" and 2 smaller ETCs with Expiration Dates in early 2023. Two days ago, I booked flights for December 2022 using PART of the FFC ($1,100 of $7,600), because it has the earlier usage date. Then yesterday, I walked through booking international flights for September 2022. The fare would be more than the FFC. At the Checkout screen, I discovered I could apply EITHER the FFC OR the ETCs, but not both types. "Please note, future flight credits can't be combined with travel certificates. To use multiple credits, select either all flight credits or all travel certificates to pay for your booking." Aargh! Foiled again! I bailed out on the transaction. Then I cancelled the $1,100 flight that I had paid for with the FFC and rebooked using the ETC, so at least I'd have that amount added back to the FFC.
I thought I had read here that one could apply BOTH FFC's and ETC's to a booking? Apparently not, unless perhaps you can do by phone, which I did not try.