I generally try to discourage splitting the party. Sometimes I'll outright say "if you really want to do this, you can, but we only have X amount of time left this session and I'd prefer not to have to manage two separate groups for it" or with larger tables, outright say "no, you can't split the party; I can't be focusing on two PCs while six others sit there not doing anything, so you all need to decide on what the party does as a group."
But it still happens. When it does, I generally try to move as quickly as I can to get the party back together or move back and forth between the groups as quickly as I can, minimizing the time people are sitting there not doing anything. If it's scouting ahead, that's one thing, but if it's just "I want to go do this thing without the other party," I will frequently have the main group of PCs do their thing first, and then get to the person that wanted to go off on their own.
I will sometimes move into initiative order, even if we're not in combat, as a way to try to make sure everyone's getting even time.