Subaeratus is Latin for "bronze/copper underneath", and would have been the/a
Roman way of calling this
type of coin. The word is used by Persius in
his Satire 5 ("nequa subaerato mendosum tinniat auro"), so in this instance used for gold, not silver. I don't think it really mattered how you hid the core, just that it was beneath the layer of precious metal - as opposed to
counterfeits made by adulterated metal.
I have encountered "
subaerat", without the Latin -us ending exclusively in
German so far, where I haven't come across "fourré" yet, but that may be lack of experience.
Fourré (or fourrée in the feminine form) is
French for "stuffed" ("crammed" or "filled"): the silver/gold coin is stuffed with a baser metal. Fourré seems to be more frequent in
France, obviously, and in English-speaking countries. Again, this latter comment is based on personal experience, and may thus be unreliable.
In conclusion, I would hold both to mean the same thing. Gold/Silver above, baser metal within.
The usage depends perhaps on which language you are
writing in, and, in languages besides
French and Latin, whether you'd rather borrow a word from the
French or from the
Romans, or use a homegrown one - ("
plated", perhaps, for the English speakers. Unless that means something different again).
Edit: of course, we can always through the Greek
hypochalkos (like subaeratus, literally eaning "copper underneath") into the mix.