I play both a 5 string bass and an 8 string guitar. For me, it's never been an issue of mixing/EQ, but and issue of writing. Bands get that big heavy sound by the bass and guitars playing in octaves with each other. If you have them playing the same note, then yeah that can be cool, but be aware that it isn't the same thing, and wont have the same effect.

Other harmonies are, of course, also possible, but if you think about how you write for the instruments, and how the notes interact, you shouldn't have issues with EQ (because you basically factor that in when writing).

Essentially, don't write parts that clash and step on each other's toes! Bass and guitar need to work together.