Is it possible to get wider apertures, like f/8, if we stack two 10-stop ND filters?
Absolutely. Let's assume you're shooting on a bright sunny day at a sunlit subject. Then by the Sunny 16 rule, At ƒ/16, your shutter speed should be the inverse of your ISO. Therefore, if you're using ISO 100, then your shutter should be about 1/100 s.
However, you want 30–120s, which is about 11.5–13.5 stops more exposure than 1/100s (that is, log₂(30 ÷ 1/100) = 11.55).
Now, when you put on your 10-stop ND filter, 10 of those 11.5 stops are accounted for. Stopping down from ƒ/16 to ƒ/22 takes care of another stop, so you are now only overexposed by ½ stop.
But you want ƒ/8, which adds back another 3 stops overexposure (from 30s, ƒ/22). At the 120s exposure, you're at 5.5 stops overexposed. And if you add on another 10 stop ND, you'll be 6.5 stops under-exposed at ƒ/8, 30s, and 4.5 stops underexposed at ƒ/8, 120s.
Now we can see that not only will that 10 stop filter allow you to open up your aperture a bit, it will require you to either open it up more, or dial up the ISO, or some combination thereof, by about 6–7 stops at 30s, and by about 4–5 stops at 120s.
Again, this all assumes Sunny 16 exposure. Your conditions might be slightly different.