Order Unit

We have a business requirement under 2 sites (plants) in the same purchasing organization to buy one article (material) from one vendor using two different units of measure. We can do this manually by setting the variable order unit in the purchasing info record, then manually specifying the particular order unit on the purchase order during creation. We have the requirement to create the purchase orders via interface, where can we store the default order unit for one site vs the second site? The default Order unit is at article/vendor level, no purchase org, or site (plant) level default for the unit of measure field appears to be available.