Bazyle –
Using two horizontal milling-cutter is not implausible because the smaller diameter ones can be mounted on a vertical-mill arbor.
The method also has the advantage of neutral torque whereas using a single cutter can risk rotating the work-piece.
That risk is greater if the work is held vertically via a threaded (e.g. Myford) chuck on a rotary-table. (As I once found when trying to mill a polygonal end using the jig-borer.)
Julie –
Rarity of arbor-spaces: a very good point but they are not too difficult to make to sufficient accuracy for all but the most exacting work.
Apart from conventional turning from stock, an alternative approach is to use gauge-plate or similar. For some applications they could be rotary-sawn from stock mild-steel plate or sheet, then tidied on the lathe. The bore and o.d. are not too critical. Another possibility is a scrap machine-part, e.g. a bearing-race or turned collar, of the right size – I use such parts in work-holding on the mill.