The L2 assignment is blocking like old NL assignment: If there are a walker and a stunner under the cursor, and the game prefers the stunner, clicks do nothing.
With the new priority code though, cases like this should rarely (if ever) happen, unless none of the lemmings under the cursor can be assigned the skill in question.
The only case I can think of is when something other than the lemming's current action blocks it (for example, if there are two lemmings under the cursor who are both walkers, one standing on steel, one not, and you try to assign a digger).