It's a little counter intuitive at first, but addParallel() commands run in parallel with the first addSequential() AFTER them.
Code:
addParallel(1)
addParallel(2)
addSequential(1) //All 3 commands run
addSequential(1)
addParallel(1)
addParallel(2)
addSequential(2) //sequential 1 will run, then parallels 1&2 will run with sequential 2