when you are working with someone you will always be slower than you would be on your own.
When you work together you pause now and then to chat, or you will both talk to the customer and so on.
When I employed lads I soon learned to separate them if I wanted them to do well.
As has already been mentioned, other factors have to be considered, you CAN be very efficient, but it takes focus and planning.
there is little can be done with regards driving between accounts, but if one of you is doing upstairs, once you have finished the tops you don't drop down and help finish off the bottoms, you move straight on to the next account and let the other guy do that and also do the collecting and the writing out of any invoices.
If you have to finish a house together, while one of you collects, the other loads up the van so you are ready to drive straight off to the next account.
Or if there are several houses you simply split up and don't work together on the same accounts.
Setting daily targets helps too, but two people working together out of the same vehicle are highly unlikely to do twice as much work as just one person working alone.
Ian