Why don't you write up an offer with owner carry at their price for 1 year, then you can do a straight refinance into a conventional in six months, using the other units rent to help qualify.
Some agents may frown upon this approach, either because they don't understand it, or are afraid they'll lose their commission, but if the house isn't selling, its time to educate them on getting creative.
If you could put less down with the owner, it might allow you to put the rest of your cash into improvements. This could bring in a higher appraisal at refi time, allow you to possibly raise rent in the other unit. You'd have to run the numbers, and I'd suggest talking to some local banks for this type of refi, but it is doable if the property has potential. The hardest part would be convincing the owner and agent, part of which is offering attractive terms for everyone.
If the numbers don't pencil, keep it in mind for the next deal, as there's always another one around the corner.