The first rule of D&D is “don’t insult the DM’s mother.” The second rule is “don’t split the party.” It’s generally considered the worst thing you could do in the game. There are a couple reasons for this. The first is the awkwardness of trying to run a story for two separate groups. It’s hard to avoid sharing information and meta-gaming, plus there’s the matter of boredom when your group is not at bat. Then there’s the potential for getting your butt kicked.

If the DM doesn’t hold back, half the party might trigger an encounter meant for the entire party, which will squish a half-power group. Even with a scaled back encounter, the party is not at full strength, lacking skills other PCs bring to the table.