2 on same house I would say about 60% more work maybe 65% at a push.
Some one has already said working at one end meeting in the middle rate will go up to 90% maybe 95% and competition will start which is great for you.
I have tried every way possible with staff and them being on there own IMO is the best by far, working as a team but sepertate is next best working together on a house is the lowest.
thats wfp or trad.
I have seen 5 guys before cleaning one house :

they was quick I will admit, but 2 us of did near on the same work as they had to wait for payment were we just ticket and let them send the payment cuts out alot of time during the day.
Ian