I need to throw a question to the crowd. [UPDATE: POLL ADDED]
If you look at a reference work such as A Manual for Writers, you will find that the plurals of prepositional-phrase compounds follow the rule governing the first noun of the compound:
brothers-in-law commanders-in-chief men-of-war
Ok… here is one for everyone. What do you do (oh, is this a straight line) with …
gin and tonic
Some contend that the plural ought to be
gins and tonic
and some say
gin and tonics
The idea here is that a) the gin is the most important element, so important in fact that it must receive the emphasis – ubi maior after all – or that b) the gin and the tonic together form such a perfect synergy that the general rule must apply.
Let’s get to the bottom of our gin and tonic question.