In high-level terms, your report should be based upon a query that returns only the two vehicles that you want in the report.
Since you will have multiple criteria (2 vehicles), you'll have to design a form that will accept whatever you're using to specifically identify the vehicles and then pass those values to the query.
If this is a one-off sort of thing, it might be easier just to code the query to return the two vehicles you want and then run the report from that.