When you attack a hex, the game will use all available adjacent friendly units. Often that is what you want to do, but if you do want to split your assault think carefully about putting the right units in the right places first.
Infantry
Infantry in well-entrenched defensive positions are almost impossible to defeat without overwhelming force, so do not even try. Soften them up with artillery while massing forces for an assault.
Offense
When on the offense, it is easy to forget that the enemy also needs high odds for a successful counterattack. So it is safer to leave lone units unsupported than you might imagine.