The effects of multiple Boon Reflections are cumulative. For example, if you control two Boon Reflections, you’ll gain four times the original amount of life. If you control three Boon Reflections, you’ll gain eight times the original amount, and so on.
In a Two-Headed Giant game, only Boon Reflection’s controller is affected by it. If that player’s teammate gains life, Boon Reflection will have no effect, even when that life gain is applied to the shared team life total.