Well 2 men working together, even once fully to to speed will not generate the same turn over as 2 men working separately...it's a time and motion thing.
however, just how much they will do is going to depend on your prices for one thing and how well organised they are in their method for another.
If - for the sake of argument - doing the same work they will be doing you can turn over £200-250 a day, it would be unrealistic to expect them to do the same per person, but they should be capable of getting within 25% of your own figures. (per person).
How much is impossible to say, but if you are going to be employing people, all legal and above board, and supplying them with a van and WFP equipment, then they are going to need - once trained and up to speed - to be turning over £200+ per day....thats an awful lot of work.
If your round is of the kind that realises £100+ per day (just you by yourself) then you would not benefit from the cost of investment and running costs involved in employing.
Of course, if you already work as a two man outfit, (and you seem to indicate that in your post) out of a single vehicle, then it is reasonable to expect that if you have a new guy working with him (once up to speed) to to be within 5-10% of whatever you can do.
You are the boss, so you are going to be pushing harder and setting targets when it is you doing the work, so you can't expect employees to have the same level of motivation as yourself.
To be honest, there are so many variables that you need a lot more knowledge of your round and the returns it already generates to know what kind of returns you can expect if you send a pair out unsupervised to do the work.
Presumably you must have very well paid work already and plenty of it or you wouldn't be considering it!
It will be interesting to read others take on this.
Ian