Don't believe that is possible, for the reason you listed. Hard to determine the cash value.
I have not heard nor read of this as an option anyway.
(I could see if you had two guests, and 2 credits used on one person, while the other person rec'd a bill to pay at the end of the meal.)
OT, but just a warning: If you happen to be doing a split stay and want to dine at a Signature on the last night of your first stay, you also can not combine one credit from the first stay and one from the second stay either (even though both sets of credits will be available to you that day). They can not pull one credit from your first room key and one from your second room key. Computer can't do it and manager can't overide it. Don't ask me how I know.
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