The low pressure supply is before the filters so the water going to the pressure washer would not be pure.
The advantage is only there if you have two operators working from one van with a pump each. Most guys on here seem to be one man bands so would not come accross this problem.
If you have two operators using an on-demand system then you usually need two filters in series for each pump. If you could have a low pressure pump to put the water through the filters first, then you could connect all four in series, this has the effect of extending resin life.