In a high technology setting, there's little excuse for PCs to ever be truly out of contact with one another. The simple expedient of a two-way radio or cell phone or whatever solves issues of communication when the characters are separated. The trick then is to come up with plausible excuses for why the communication might stop working: out of range, dead batteries, smashed equipment, locked in a radio-proof metal room with spikes descending from the ceiling and water rushing in...
You just have to be careful not to use the same means of failure too often, lest it become cliche and unbelievable.
The other option is to let the characters communicate, but still get into trouble anyway. It is a good way to arrange rescue scenarios, for example.
Han: Princess, ah, there you are! You gotta come with me.
Leia: What are you doing? Let me go!
Han: I'm getting you out of here to safety.
Han: You may be right about a traitor inside the Rebellion. They tipped off the Empire at Yavin 4, Thila, Teardrop, now they found us here too.
Leia: Wow. You don't say.
Han: I'm the only person I trust.
Leia: Okay, fine. Let's go.
Leia: Luke, hang on! We're coming to get you!
Leia: General, we have to evacuate, now!
General Rieekan: You go! I'll be damned if I let any droids get in here!
C-3PO: What, I don't even count now?!
General Rieekan: Well someone's gotta fix me my salads.