One option might be to make it a Flexible assembly. One slide of the pair could be constrained to the origin, and the other left unconstrained. Then set the subassembly as Flexible in your main assembly, and constrain as though they actually were separate parts. This would probably be my preferred way of modeling something like this.
Another would be to just insert the two parts separately, but have one of them set to the "Phantom" BOM structure. The "Phantom" part would not show up in the BOM. The one left as "Normal" BOM structure would carry the part number of the purchased assembly.
Although my usual preference for something like this would be the first method, I have used the second in situations where the two parts needed to be at different levels in the assembly. Think of something like a toolbox, where you have:
- Weldment for the body of the toolbox.
- Weldment for the lid of the toolbox.
- A draw latch, with the latch body welded to the box, and the catch plate welded to the lid.
- Draw latch assembly is sold as a single purchased assembly.
In my case, I used the main body of the latch (which is the bigger piece) to carry the part number, and set the catch plate as Phantom.
