Sure, the rivalry is increased (which also means the fighting can intensify after too much time together). The individuality and the individual attention by either the coach or the parents may be lessened. And the kids might just want some space away from each other to form new friendships outside of each other's inner circle.
However, it only makes sense to request that they play on the same team, take lessons together and go to summer camp together if these are sports they both like to do. Easier for mom and dad. Fewer nights and weekends juggling our schedule to attend games. Only one coach to communicate with throughout the season of that particular sport. Similar gear to buy (even if the colors are different!). And maybe, just maybe, our kids would even grow in their sibling relationships with one another, where fighting actually subsided and they started to work together as a team, knowing that their bond and their competitive drive was and should be greater than anyone else on that team or playing surface.
With all that said, however, kids do need their own time and own sports and own individuality. We found that out when our girls asked to play sports like ballet, ice hockey, swimming, and lacrosse that the other sibling didn't want to play. And so we signed each girl up without the other, and we watched them grow into their own selves with their own sports interests and identities.
The other time when it may be good to sign kids up on different teams in the same sports league is if you do have time to nurture them and attend separate games, and you do want to experience a sport to the fullest. There is nothing like watching your kids compete against each other. That builds character too (and not just character of the kids)...a friendly competition within the family! Be prepared for tears and jealousy and frustration (all kids experience it), but also know what the handshake after the game means to two siblings. Again, a bond like none other.
Like us on Facebook at www.facebook.com/besporthappy or join our Be Sport Happy Saratoga Equipment Exchange group on Facebook and exchange used sports equipment with other Saratoga families: www.facebook.com/groups/besporthappy.