You probably got a second voucher created ( possibly an "02" voucher), because the cash account on the new fund is different from the old fund. 

Since you can't cut a check for one amount on two check stocks at the same time, FIMS will automatically create two vouchers.  Each voucher will have the portion associated with each cash account.  Thus, you can cut the amount coming from one bank/fund on one check and the other amount on a second check.

If you want this to be paid on one check, then follow these instructions to change the cash account on the posted voucher:  How do you change the cash account on a posted voucher?.  Once modified, you can pay both vouchers with the same check.