The short answer you are looking for is this: if MFS and BOTH PARTIES AGREE, then the overpayment carryforward may be split in any manner they choose, and the IRS will accept that allocation. This is done by each filing a return claiming their agreed-upon share of the carryover. Absent both filing or If they can't agree, then the IRS will allocate based on a formula that divides based on each person's proportional share. Cite is reg sec 1.6015(b)-1(b)
Also, the IRS may still question and reject that POA even though it includes "tax returns" because IRS wants its POAs to include the very specific language found in the code.
Sorry if that isn't much more help on how to file. At the risk of being rejected, I think I'd try to file it as MFJ rather than as separate returns because at least the attention won't be drawn to the splitting of the carryover right from the beginning.
The only other thing I can think of is on a MFS return is questioning whose funds mostly created the overpayment to begin with? If it came from a separate account in husband's name, or if it was all from withholding from husband's retirement distributions or social security, I think the wife would have a harder time claiming the overpayment if the IRS questioned the MFS return claiming the carryover if a return for the husband is rejected or never filed.