Luc
As with all machines, there are entry level dual wanders. Some of these lower spec machines will only perform to an acceptable standard with one wand and one upholstery tool over modest hose lengths, so you will need to define your needs quite accurately.
My gut feeling for residential work is that the time saving for 2 wands wouldn't justify the capital outlay. Because of hose management and set-up/breakdown times, it could conceivably be slower
How many of your jobs a day/week are housefuls that would possibly justify the two wands?
If your 318 has only one solution outlet, try a line-splitter and your assistant can use an in line sprayer whilst you are wanding. This could save a lot of time and bucket loads of cash. If the assistant were also to agitate with, for example, an Envirodri, this too could make the wanding much quicker. Depends on your soil types and density.
Safe and happy cleaning
The Ken