By default, we use different testers if you want to do multiple iterations. It is designed that way because in most cases it makes more sense to use new testers and to get fresh unbiased feedback. If you want to re-engage the same testers we can do that in a future iteration, but we can't make a guarantee in advance about how many of them are going to be interested in participating again.
Alternatively, we can design the test so that we recruit the testers for a multiple week test up-front. All testers would be joining knowing that this is a multiple week test. This is the ideal way to design the test if you know you will want to re-engage the same testers over multiple weeks.