We used to have 2 units in our building and shared expenses (Sacrament supplies, Library Copier supplies, etc) were confusing for reimbursements. Several years ago, the other ward's clerk and I met to come up with an easier way to handle "shared" expenses. Members were trying to catch each ward clerk to get reimbursed for 1/2 of the expense from each ward, then each ward had a reimbursement check for members of the other ward they had to chase down.
We thought it would be easier and more straight-forward for the member to just get reimbursed from their home ward. Then that ward would "bill" the other ward for 1/2 of the expense using a "billing" form. Everything is traceable during an audit (though the original receipt resides in the ward who does the original reimbursement, we use the "bill" from the other ward as our receipt and it can be traced to the original receipt if needed).
We like this form and it has made things much easier for the members and the clerks. I am attaching it here (in both editable and .pdf format) in case anyone wants to adapt it for their ward. We now have a singles branch in our building as well so I recently updated it for 3 units.
Note: there are 2 forms on each page - print, then cut in half along the horizontal line.