I think someone above explained why sometimes you can use multiple FFCs and sometimes not. I originally thought I would be able to use only one and sent a detailed DM via Twitter to United with all the PNRs that had FFCs attached and asked them how I can combine into 1 or book using all the values. It took them too long to respond and I needed to book for the following day so I figured I'll just use the largest FFC and worry about the others later. I was going through the process on my desktop and was surprised to be given the option to add another FFC during payment and then another.

To be fair all these FFCs originated from 1 single PNR but then I remained with smaller ones due to cancelations and rebookings of subsequent reservations. So maybe that had something to do with it.

To clarify what I mean by that, I booked a flight for $1200 but couldn't take it so got FFC. I used it to book a $700 and had $500 remaining. Then I booked a flight with the remaining amount for $450 (so still had $50 left) but that flight had a weather related cancelation and I got the $450 in FFC but attached to the new PNR. So now I had 1 x $450 and 1 x $50. This happened yet again (booked $445 flight leaving $5 and that $445 flight was canceled and FFC attached to new PNR), so to my good luck this resulted in me having 3 separate FFCs (50, 5, 445). But essentially they all had the same "parent" so maybe that's why in my case I was allowed to use multiple ones as payments. I first used the largest amount and then added the smaller ones and paid the remainder by cc. Flight already taken so I'm glad to be over with this.

In my opinion they should have just kept putting value back to the original PNR which would have left me a lot less confused. But I'm guessing that technically isn't possible due to having booked a new flight with new PNR.