Never separate your squad too much in XCOM unless if it's for one of two reasons.

1. A squadmember is critical and close to dead. In which case keep them strictly further back in already scouted areas.
[u]NEVER[/u] send anyone out alone into un-scouted territory, especially someone close to death.

2. Setting up an ambush/flank.
As in distract them with group A so group B can get them from the side, or attack from the roof etc.
(I've had cases where they completely miss the soldiers hiding on the roof above so my men could just jump down and get flank shots on all of them from behind).
But even with this [u]proceed with caution[/u] cause this plan quickly backfires if one of the two squads get's the attention of even more aliens.