I don't know what the written rules for placement are, but most small sailboats seem to have their masthead (steaming) lights about 1/2 way up the mast. My 26X has it's a couple of feet below the spreaders (about 1/3 of the way up the mast), but my Hunter 340 had it's masthead light at the top of the mast, and when the back lens was lit, the whole unit became an anchor light.
If you have a multimeter, measure the DC resistance across all the leads - three sets of resistances. The highest resistance reading will be from running both bulbs in series, so those two leads will be the hot ends of the two different bulbs. The third wire is the common (battery negative), and there will be a lower resistance reading between it and either of the two 'hot' leads. The hots are also what you switch to control the two independent bulbs.