Brendan
From speaking to respected colleagues who own CFR 400's, there's no problem with single vacs. The only limitation is not on performance, either rinse or extract, but the length of hose. I'm led to believe that it's advised a max of 70 ft for a single vac machine and about 130ft for a twin vac, but I stand to be corrected. 70 ft is plenty for any portable, I rarely use more than 75ft with my van mounted potty.
I gather that with CFR, the wand came first. It performed exceptionally well. But on a portable the water consumption was too great. So they designed the CFR around the wand. At the recommended hose lengths on the 400, the extraction is barely improved, if at all, with a twin vac. They even have models that run at 700 and 1000psi on single vacs
I've heard of a couple of cases where a vac went down on the 500psi model. It was disconnected on site and run as a single with no difference in drying! I was told that the twin vac is in the UK for marketing rather than performance reasons (ie us Brits can't get our heads around it) In the States, our twin vac CFR is sold as a single vac with a bigger pump (700psi) which they say is a better. performer.
Safe and happy cleaning
Ken