Hi,
When you have two fields with the same target type, you would do exactly as if you have a single property. This is a part we have yet not put in the docs (coming very soon!), but the UI would help you create this transformation. But to also answer your question, assuming you have a RAW table with the columns frontPart, backPart and externalId (the first two would be external ids to the Part instances)
Select
array("datamodelexternalId", `frontPart`) as `frontPart`,
array("datamodelexternalId", `backPart`) as `backPart`,
string(`externalId`) as `externalId`
from `database`.`table`;