I would say there are three ways to do it. Common for all methods is that you need to exclude the two common field from one of the tables off you want to keep them in the datamodell without having a synthetic key.
1. Creating a field by combining the two.
Date & '|' & Dimension as %KeyField
2. Auto-numbering the unique combinations.
Autonumber(Date & '|' & Dimension) %KeyField
OR
autonumberhash256(Date,Dimension) as %KeyField
OR
autonumberhash128(Date,Dimension) as %KeyField
3. Hashing the field combinations
Hash256(Date,Dimension) as %KeyField
OR
Hash128(Date,Dimension) as %KeyField