I have two BOMs, each with the same unit of measure. When I add a line to BOM1 of type Production BOM that reference BOM2, I get no errors. Where I'm seeing the issue is when I do this through the API, passing in Type as "Production BOM", returns the following error:
"Type must be equal to 'Item' in Production BOM Line"
We are only using one unit of measure in our environment, I've seen that typically be an issue in the past, but I can't see that causing any problems here.