Does it make sense to own both the DX 35mm G and DX 40mm G ?

It's not nuts to have both lens in the bag!

1) For $199, the 35 1.8 G on a DX camera makes a D5100 a sharp, fast shooter. You get the benefit of the 1.8 in low light/indoors. The 35 makes a nice 52mm FOV.

2) For $279, the 40 2.8G macro is a sharper if a step slower lens for everyday use. The 40 makes 60 FOV on a DX camera. It also focus much closer. Best use as a macro is as a copy lens & non moving subjects. As you focus closer, F goes up to F4 so you will need plenty of light & sunny days for the best results. The working distance means you also need plenty of sidelight to keep the camera/lens out of the way. You could use a setup where/when you get direct light from the sun into the window & on your subject.

As a combo you get 2 very different lens that share the same 52mm filter at a very similar size & weight. I bet just because of the price, you can get the little 40 & have some fun poking a lens into anything on a sunny day. Just be sure the bug or something has nothing to get mad & bite back. for those kinds of critters, the bigger the macro the better, but you can't get it new for less than $300.  by then, you can step up to the Nikon 105 2.8 macro when you can afford it.

If you do bugs & other stuff, the 85 3.5 makes a nice companion on a DX body. You can also try the Tamron 90.

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I Shoot RAW