Answer
Yes — a user can be granted any number of LTU permission sets, and as long as every permission set they hold is LTU-type (with LTU permission sets only able to build on base runtime permission sets), the user is still considered an LTU user overall, not a Full User. Combining multiple LTU permission sets is the supported way to grant a user several distinct limited-task capabilities without needing to license them as a Full User.