This segulah practice is generally intended as a sort of fertility aid to the woman who immerses second. Its significance is spiritual and not halachic, so there is no halachic limitation to your immersing as long as you are tehorah. That being said, you might consider disclosing to your friend that you are using contraception if you feel that might change her motivations. But making such a disclosure or declining your friend's invitation is not halachically required.