Allocate billing amount across both TIME and DISB amounts

I know there is a similar entry for scheduled bills, but this is for standard billing process - we would like the ability to allocate a bill amount against both TIME and DISB entries combined as quite a few of our projects include any disbursements in the fixed fee.  Something like a checkbox so that other projects that do track time and disbursements separately are able to, but we have the option to combine them in billing for others.