OK, what you should do is half fill one of the buckets with sand, top up with water and swoosh it around a bit. You'll see the dust come off the sand, so your pour that away (maybe best to do that outside, rather than an internal drain). Top up with water again, and swoosh it around a bit. You'll see some more dust come off the sand, so you pour that away (maybe best to do that..... ok you get the picture). After several top-ups and swooshes, you'll see there's very little dust coming off. When that happens, pour the sand into your other bucket.
Repeat with the next batch of sand.
And the next.
And the next.
And the next.
Until you've got enough sand.