Currently I am using a Hoya ProND 1000, 58mm ND filter on a Canon EF-S 18-55mm kit lens for long exposure photography during the daytime. But even after using this filter, I can't use wider apertures and I have to stick to f/22 during the daytime. Is it possible to get wider apertures, like f/8, if we stack two 10-stop ND filters?
Yes, of course. Filters add up in both effects and defects. The more ND you stack, the less light will reach the sensor. Stacking a couple is usually fine, although you increase chances of flare for each one you add. The added thickness also can cause vignetting depending on the lens and filter.
For simplicity, image you have a scene which exposes at 1 second and F/16. When adding a 2-stop ND-filter, you can exposure for 4s at F/16 or 1s at F/8 or even 2s at F/11. If you were to stack another 2-stop filter, you could expose for 16s at F/16 or 4s at F/8 and so on.
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.