Can I get away with 2 ND8's on an 17mm lens?

I was wondering if I could use 2 ND8's on a Nikkor 17-55mm 2.8 for DX, using the wide end? Thats crop factor of 1.5

If danger of vignetting I will get a stronger and and just use one, but the 8's are the strongest I have at the moment.

Seeing that you already posses the combination I suggest that you make a few tests. Experimenting in digital costs nothing. Try all aperturtes, since vignetting shows up differently at different apertures. I assume that you are trying for very long exposures in bright daylight - an 8x on top of an 8x is the equivalent of a 64x; you multiply the values, not simply add them. But anyway experiment with what you have already got to see if the typical exposure times are what you want, then decide if you need to buy something else.

Without knowing how slim the mount is on each filter it is impossible to answer your question definitively - the odd millimetre of thickness will make a difference at 17mm on the lens - if you zoom in just slightly, say to 20mm or 22mm, you will probably avoid vignetting.

Your concern about vignetting is easily answered by trying it out - but I would be just as concerned by image degradation, which a few simple trial photos may not show. The more glass surfaces you add to your lens, the greater the potential for ghosting, internal reflection and distortion. Vignetting you can see through your viewfinder, but other flaws induced by the extra surfaces may not show up in all conditions, will be difficult to see until you get the images onto a big screen, and may not be easy to identify, since while some flaws will be obvious, some are going to be quite subtle losses in contrast or saturation.

I'd be surprised if you don't get some vignetting. But as said above, it doesn't cost anything to try if you already have the two filters.

One thing you have to understand, that when adding both filters, the classification of the filter can be confusing.

An ND8 filter is also sometimes known as an ND0.9 which can be confusing. At any rate, the filter adjusts the light input by -3EV, or 1/8 of the light (which is where the ND8 comes in).

When you add another ND8 filter, the adjustment will be -6EV, or 1/64th of the light (Known as a ND64 or ND1.8 filter).

It all has to do with the inverse square law.

Personally, I have two and filters; a ND16 (-4EV) and an ND400 (-8.65 EV). I have found these two densities work well for my "silky" waterfall needs, etc… They are both 77mm which means they are pretty expensive, but I use step down rings to fit my various lenses, and I don't stack them.