Do you have 1st lien on the property? If so then it becomes difficult for HELOC.
In regards to investment property; even when you dont have 1st position loan; HELOC is less common from regular financial instituation.
You could possibly try a local private lender or hard money lender who knows the area well. They might be open for HELOC on multi-family.
HELOC on commercial usually involves an open ended mortgage recorded. There may be minimum draw amount, draw fees each time you make a transaction, minimum number of days for interest commitments-to return back the funds.