If you use a #8 then the crossing angle has to be difference for it to come out straight, as the #8 is a shallower angle. The angle is arctan(1/frog number)
Or do you mean just connecting a pair of #8's diverging route to diverging route as a single crossover between two tangent tracks? Yes, this works fine. All of the numbered turnouts have straight diverging routes, not curved. The difference is what the center to center ditances comes out to, and how long the whole setup ends up being - with the more gentle angle, it takes more linear space to get from one main to the other - but then it also means the whole crossover is less of an S curve and easier for longer locos and cars to handle. Like everythign else, it's a trafeoff. I'm using #8's for my main crossovers, but #6 most anywhere else, since with 30" radius curves, a #6 is plenty big enough to not be the bottleneck, and they are significantly shorter than a #8. It just better fits the length of run I want in my space to use the #6 for turnouts off the main, and while I technically don't need #8's for anything I run to cross over (I has #6's two layouts ago, and my 4-8-4 would run through are ridiculous speed with no problem, and most of what I run is much shorter - 4 axle diesle, 40 foot cars), it does flow nicely and looks betterI don't have a lot of crossovers between mains, so I'm not 'wasting' too much space. If Peco changes the #8 the way they just changed the #6 - it will really look good.
--Randy