How do I split up my party without causing issues?

My players have split into two different investigation parties in our most recent session, but the issue is that I don't know how to make it engaging for them. I wasn't really expecting this, and tried to roll with it, but I'm scared I'll either spend too much time on one party, as 'Party 2', is investigating stuff that's far more important/story-relevant, but I don't want to make Party 1 feel bored. Any suggestions from anyone familiar with this??

I did this when I was running the Stygian Gambit adventure. What helps is if you have a map, then you can figure out where everyone is. Also, a good strategy is to cut back and forth at climatic moments, so that you keep everyone excited.

My party was caught and thrown into prison, each to their own cell scattered around the dungeon. What I did was I had them throw initiative to see which order they would act. But I didn't have them make only one action per turn. Instead, their turn lasted for a minute or two until it was the next player's turn. That way they could spend for example ~20 seconds trying to listen to approaching guards or other voices before they attempted to pick/break the lock, and they would still have time to get a look what is behind the door before their turn was over. Eventually the party members found each other and became a split party of two each. Once two party members found each other, they shared initiative from that moment onward and they shared a turn.

thanks for the advice, I'll try this out. 

Quote from BruenorBattlehammer1277 >>

My players have split into two different investigation parties in our most recent session, but the issue is that I don't know how to make it engaging for them. I wasn't really expecting this, and tried to roll with it, but I'm scared I'll either spend too much time on one party, as 'Party 2', is investigating stuff that's far more important/story-relevant, but I don't want to make Party 1 feel bored. Any suggestions from anyone familiar with this??

 
Yeah my Monday night group laugh in the face of 'don't split the party'. The way I try to handle things is to shift the spotlight equally. So, spent ten minutes at a time (or however long feels engaging just split the time equally) before shifting the spotlight and finding out what the other party are up to. 

In one session I literally had two player characters head into a Beholder's Lair while two other player characters spent the same time sitting in the garden of a tavern watching the butterfly that kept flitting between flowers, and just enjoying the peace and quiet. The players all seemed to enjoy the events of both parties they all got to do the things that they wanted to do and it made for some absolutely wonderful moments. 

The other tack you could take is spread out the clues. If you've intended to find a satchel monogrammed with a suspect's initials and a signed letter to be found in a location - split those items up so they're each found in a separate location. In my view this doesn't tend to work as well, but it can work. 

Try to engage everyone and be a little more lenient than you'd normally be. Let group one pull off some shenanigans that you might not normally, let them have plenty of laughs while group two digs up important info that makes them feel like they're doing a good job.

Well, there's a reason "don't split the party" is a maxim. In general your solutions are

1. Alter the story to make the 'less important/story relevant' path more important or interesting -- maybe certain pieces of information are available from multiple sources (though this gets into quantum ogre territory), maybe you just add something to the story.
2. Make it clear quickly that this is a dead end, and let them go do something else.
3. Introduce something entertaining but not plot relevant (as long as the players are having fun, forwarding the plot is entirely optional).

I agree with @LinkArcher. It has worked well for me. 

I am wondering how your question morphed from What Issues can I cause my Party When they Split Up? :). I don't set out to kill ANY PC, but if you split up, I'll nudge more.

Quote from JohnnyC1971 >>

I agree with @LinkArcher. It has worked well for me. 

I am wondering how your question morphed from What Issues can I cause my Party When they Split Up? :). I don't set out to kill ANY PC, but if you split up, I'll nudge more.

 The more you split unnecessarily, the more likely you are to “accidentally “ run into party difficulty adventures…

As a player I hate splitting the party unless someone is trying to do a stealth mission or something. My players did a couple that turned out very well, but it fit the story and ended up being a lot of fun... but I was prepared for them.

 I some how did this well with my party during a trial. 

The set up is, my party came into this old west inspired town, caught the wanted bad guy and one of them was named Sherif. The bad guy goes on triral, during which the party finds out his gang has kidnapped the Judges grand daughter. If the trial is stopped, or the bad guy is found guilty the granddaughter is killed. 

So the party spilt, 3 stayed at the courthouse, the bard  (playing the part of the prosecution), the fighter (the sheriff), and the sorcerer because she was a witness. The other 5 players (I have a big party, it's a store-sponsored game) went to the old mining camp to rescue the girl. The actual combat was kinda tedious, because they were level 6 going against bandits, but I contrasted that with flipping back to the courtroom every few rounds so the bard could be entertaining, as they stalled for time basically, it actually made for a interesting session or two where both groups got to kinda stand out.