If she does eventually switch to a ported bit, it is best to be LOW ported, because the port depth/height will add severity.
If she is going switch bits and ten teaching to neck rein would be incredibly confusing for the horse and take almost twice as long. I thought she was going to train on the snaffle and then change bits. It doesn't work all to differently because it still has a lot of similar design including both shanks, and a broken mouthpiece.
Argentine snaffle or a Tom Thumb are a lot alike, more than you'd think. Tthose bits are more severe than a similar solid mouthpiece curb because of the nutcracker effect they can have on the mouth when you put any pressure on the reins at all. I would rather use a short shanked grazing curb bit, or even a regular snaffle as we already mentioned for training a horse/continuing training with a horse.
edit- I just wanted point at the that the bend in the bit REDUCES nutcracker effect but does not eliminate it. You would have to have a third section in the mouth piece to get rid of the nutcracker.