Can you make the meringue while the lemon cooks?

If you are asking, can you whip the eggs whites while the filling is cooking on the stove top, I would advise against it. First, the filling requires constant attention unless you're cooking it in the microwave oven. Second the egg whites have a smoother texture if spread on the pie as soon as they're whipped. One last piece of advice, put the meringue on the pie while the filling is piping hot and then bake the meringue. This starts to cook the meringue underneath and prevents that nasty slick coat that sometime forms between the filling and the meringue.