A parent cannot be a helper for two different students at the same time. They can only be a helper on one student's account at a time.
With that being said, another parent, guardian or trusted adult can be a helper on account if a parents has more than one student running a fundraiser at the same time.
Happy Helping!!
(Please note that MoneyDolly is working on creating a better experience for parents with more than one child in the same fundraiser. Stay tuned for updates!)