There are different strategies, but splitting up for faster exploration is generally better, yeah. What exactly are you struggling with? By alarm level five you want to be done with the objective and on your way to the exit already. Bear in mind you don't need to 100% every level and it's often smarter not to try. That one last safe in the far corner of the map behind three cameras and a turret is usually not worth it, etc.

(That said, it is perfectly possible to survive on alarm level six indefinitely once you're familiar enough with guard AI and manipulation methods.)

Thanks Hekateras, for the tips

Agents going out separately obviously lets you cover more ground faster, but this puts your agents in more danger being alone. You have to adapt to the current situation and make your strategic choices about how you want to split up agents and deal with enemies and objectives.

I would recommend learning guard manipulation tactics if you don't already know them. This video does a pretty good job explaining it. If you only learn one tactic though, it should be sprint toggling.

https://www.youtube.com/watch?v=FP-Bbyu-8MI