OK - progress of a sort.
I now have two printers (6P and 6MP postscript) under the same icon in printers and devices. Only one name is visible until you select an option like Printer preferences when both become visible.
Both are shared under different names; both are visible in applications (e.g. Word) and both actually work!! (though I have no easy means of determining if one is printing PS and the other PCL)
Now to the R60 on the network. This has no HP printers attached so
Add printer - network printer and both 6P and 6MP postscript are visible (!) and amazingly (!!) both work.
I say amazingly because I've been here before and NEVER seen this. I clearly must have done something different though heaven only knows what!.
Anyway, for anyone else interested,
it seems yo only get a second USB if yo uplug in (what windows thinks is) a different USB printer
So to point two printer drivers at a single USB (and hence a single printer), the process seems to be
Host Machine
1 Plug in USB printer and let it install so a USB port Call this Printer A Check test print works
2 Add printer B, select the printer you want, use any port you like (e.g. File), complete installation
3 Got to printer B properties and change the port to the same port as printer A.
4 In devices and printers the name may have changed but when you select properties you should see both names (A and B)
5 Check test print works for both.
Provided you have shared both printers, both should be available on the network and one, the other or both are available to be added to an other machine on the network.
Sounds so easy - why did I waste a couple of days of my life figuring this out!!
Thanks to those who helped