I've been dwelling on this all day, so just opened up a new tandem cylinder to check.
The smaller of the two unions is 3/8". The larger has a diameter that seems to be either slightly bigger than 7/16" or slightly smaller than 12 mm, but I've not been able to lay my hands on a bolt or union of either size to test. 7/16" seems the closer, but something niggles that there's something metric about the cylinder...
All the tandem cylinders i've had have metric at both outlet ports, I have unions in stock that fit the front (larger) port, they are the same as used in a number of Peugeot and Citroen rear wheel cylinders (which is how I buy them from the factors) I can't say exactly what the size is, i've never asked, but I can't imagine a French car company using imperial unions at ANY price! You can get a 3/8" UNF bolt into the rear port, but it will not tighten correctly if you use a 3/8" union on a brake pipe. The correct thread at the rear is 10x1.25, the standard metric male union.