I used to work in telecommunications and got to play with a survey Trimble (sorry don't remember the model). We would use it to map out network elements (thing like manhole locations, splice points, pole locations, etc.) for use with our internal GIS.
To the best of my knowledge, the unit only had one receiver, but was able to track many more satellites than a handheld unit (up to 45). It would frequently lock up on 15-20 sats. But it did take quite a while (1-2 hours). I was told the accuracy was within 5mm.