...I'm off to find a user wizard that will cut a radius from both directions someone else has to have had this come up.
From your description, you are trying to cut a Browne & Sharpe gear cutter (
http://www.precisioncuttingtools.co.uk/gifs/pic6.jpg). You are aware, I suppose, that the curve on this is an involute not a circular arc? And that the cutters come in a series of eight, covering 12-rack? And that you have to relieve them radially to give cutting clearance? I think the hardest part of your project will be forming the proper relief.
That said, it is common to approximate the involute with circular arcs.
To answer your specific question, generate the G-code for the outside diameter turning alone (and save it). Generate the G-code for the tailstock end of the cutter and save it. Then open the G-code in an editor and mirror the numbers about the Z-centre of the cutter blank. You can do this easily and manually if you set Z=0 as the blank Z-centre - just negate all the Z-values. There are programs available that will do this for you (e.g.
http://www.cnc-syntax-editor.com/).
The only caveat is that your turning tool must be capable of both left- and right-hand working (e.g. a button tool or parting/grooving tool) and you need to set the tool zero at its centre.
Practice on a bit of scrap first!
