I'm in a similar situation and have a Traffic with a 650ltr system. We had our busiest period ever over the summer but still made it work with the one van, I'm trying to keep costs down to a minimum this year and will invest in spring next year.
We do gutter clearing and pressure washing as well so need a bigger van for all the gear.
2 people in 2 vans would get more done. But it just depends how much it will costs to run 2 vans? My advice would be if you go for 2 vans get them both setup with a 2 man system - if one is off the road for a bit its a pain. But if you have only one van and its off the road for any reason thats a nightmare - happened to me earlier this year, I never want it to happen again!