It depends on how each sport impacts your grades. Some students on a swim team, rise at 4:30 a.m. for pool time as do hockey players for ice time. The demands are so great in season that, once the sport has ended students need to break from such a commitment returning their full energies to their studies. On the other hand, there are students that are able to participate in a sport year round, with no impact on their grades. It really is a matter of how the involvement effects each student, it is very individual.
There are sports such as crew and football, that likely require training year round. If this is the case for you, not sure two athletic involvements would be wise. If you are able to handle both activities and, keep your grades up, OK but if the grades drop even just a bit, it may be time to decide which to stay with and which to let go of.