my take is that you think too much.
if you can afford it, there is nothing wrong with having a camera that is unused sitting around, it is your money after all, why care what other people think?
that said, if you don't know how to utilise both properly, you get fx, dx, or both, also frankly speaking, useless. and therein lies the point.
both have strengths, both have compromises, nothing wrong with getting both. a camera, whether dx or fx, is after all, a means to an end. all that matters is the result, not what you have, and if you need more than one type of body to achieve what you want.. then go for it. of course, if it is just bbb telling you "hey, get two bodies so that i can say that i have two bodies and drool over them".. then only you would know.
there are d40 users who put d3 users to shame, just this fact alone speaks volumes.