If you have one Sim experimenting, other Sims should also have the option to 'Experiment' when you click on the cauldron. (I had all three of my Sims experimenting last night, so this definitely works.) They can also help brew a potion, as noted above - I also did this with three Sims last night. The number is probably limited by the number of Sims who can fit around the cauldron.
If your other Sim is visiting, I don't know if there is a way to get her or him to join in with the potion brewing or experimenting. You don't get an 'experiment with' or 'brew potion with' option like you do with chess or other activities. A visitor might join autonomously.