I have both the 7/16 x 10 LH ACME and the 3/8 x 10 RH ACME taps. I
bought them from:
Tracy Tools Ltd.
2 Mayors Avenue
These are advertised as "for renewing feed screw nuts". The one nut
I threaded seemed a bit tight on the screw. Probably it could be
lapped in, though. The taps are not tandem. They are finishing taps
only. I could really use a roughing tap to start the threads.
Single pointing to start and finishing with the tap would work, I
guess. Price was 18.00 pounds each plus VAT and shipping.
Their catalog says they have other sizes in stock.