Up to 2 squads/reduced squads can combine their fire as long as there is 1 attached Leader.
You would only need 1 Leader still if the units you're using are adjacent and not in the same hex.
Gents what Shayne is saying here is specified in 16-87 bullet 5. And bullet 11 in red text is a good description of this.
A re-read of all of 16-87 Fire Groups is worthwhile. I've had to do it several times just to be sure I'm straight.
- edit - to clarify... I'd forgotten this is the Vol I forum. I'm stating rule 16-87 from Version 5.5 of the rules (Vol II). Paragraphs are numbered differently than the 5.25 rules in Vol I.