Neil,
See earlier posts, by myself and Steve Boese.
Ebay links right now:
Marine unit, with 2 flow sensors:
Another marine unit (likely includes the sensor, but you should ask before purchasing):
Aviation unit:
There are numerous 'heads' listed, as well, and a brand new flow sensor is only around US $150 retail:
They're frequently available with the marine heads, or with older Shadin heads for less than US $200. I know, because I have a few on my parts shelf right now.
Watch for Floscan, Shadin (typically uses Floscan sensor), Flight Data Systems (uses either Floscan or FT60, depending on age), Electronics International (FT60 'Red Cube' sensor).
The Floscan 231 & EI FT60 are interchangeable, used with any of the common heads; you'll need to calibrate either's K factor to your system installation.
As we've said, you don't need two sensors if you 'dead head' flow to the injectors.
Charlie