To me, the boundary is set by the fact they cannot do two things at once with the Force. Using a lightsaber is not a Force-power but a learnt skill, so they can attack and parry automatically without calling on the Force. You can see that they never use two skills at the same time and this is consistently throughout the saga, not just in the original or prequel trilogy.

The biggest example of this would be Yoda's rescuing of Obi-Wan and Anakin after the fight with Dooku. If he could utilise the Force in more than one effort, he'd have stopped the ship from leaving as well as hold off the ton of bricks coming down on his head.