Can SS just tag runner who is still standing on 2nd base?
Yes. The ball in play makes that runner no longer entitled to second base. While the force is in play, tagging the runner at any point before the runner reaches third base would be a force out.
Not sure what league you're interested in, but NCAA softball rule has:
188.8.131.52 A runner is considered forced out if they must advance to the next
base or to home plate in order to make room for another runner, but:
184.108.40.206.1 They are legally tagged as in Rule 12.9.1 before they reach the
Standing on second, the runner has not reached the forced base (third) and is out.
(For a league just using MLB rules, this is found in 5.09(b)(6).)
Then touch 2nd base to force out runner going from 1st to 2nd?
If done before the runner reaches second base, that's correct.
And the order is important. If the first action of SS is to step on second base, the runner from first is out and the force on the runner from second is removed. The runner from second would then be safe at second base if tagged.