"Don't split the party" is common advice, and it is wise to heed it at times, but it has its uses and can enhance the game on both sides of the screen when done well. The trick is to not do it in a way that disrupts the fun of the game for everyone at the table. Here I am going to go over some basic thoughts and give suggestion on how to do it right, based on what I have experienced.
I think it is important to distinguish between the different types of party splitting. Going to different stores in the next city isn't the same as going to different shorelines of the continent. As a general rule, the greater the distance and time, the more likely it is to be a problem and at a certain point it is no longer a split party, but two different campaigns entirely.
Still larger scale party splits can be done if the GM is attentive and everyone is on-board. I wouldn't recommend doing it too often. It is more of a once in a while thing. In one of my own Ravenloft campaigns, many years ago now, the party needed to acquire information from an NPC who lived quite far and an object from a ruined city (I no longer recall the details of the information or object). They decided on their own it would be better to split up so they could save time. This meant a session and a half of the party being on two completely different paths.
Because it only happened once in that campaign, and because it was a conscious decision of the players, where they understood what it meant, it worked out well. As the GM, I did my part by being sure to shift quickly from one group to the other. I found early on that it was to easy to linger on one group and ignore the other, so I forced myself to shift focus more rapidly than I was naturally inclined. It could have been a disaster, but it wasn't, and one of my players still comments to this day on how great he thought that part of the campaign was.
What also made this work was the party still shared a goal. They were working toward the same cause and the information and object were linked. Where it would have been more problematic is if they had split because one group wanted to hunt vampires in Barovia and the other wanted to kill wolfweres in Kartakass.
So as another general rule, when the party splits and they share a common goal, I think it is more likely to work out.
If the GM carefully plans each encounter around the party's resources and characters to achieve a particular challenge level, splitting the party may also present problems. I don't really run games that way so much, so splitting the group hasn't been a huge issue for me (and running away is always an option if things get too difficult). Still you do need to account for your group's preferred play style and that may be a consideration here.
Understanding your own GM style and personality is equally important. Some GMs have greater difficulty juggling two different groups. It can be a bit more confusing. Personally I find it helps me clear my head and come up with ideas. If one group is heading to the morgue to interrogate the mortician and another is going to the graveyard to search a tomb for clues, I have time to come up with stuff on the fly when players do unexpected things, as I shift back and forth.
I think if you ask yourself, is the split....
1) A conscious choice on the party of the party
2) For a shared goal
3) Shortl enough distance and time to be sustainable
4) Manageable for the GM
5) Compatible with the group's preferred style of play
...and the answer to all four is "yes", a split could actually enhance play. I think the more you answer "No" on this list, the more likely it is you will encounter problems.