X-Com 1: Having a secondary weapon
Solution 1:
According to the UFOpaedia, having an item in your off hand when firing a two-handed weapon reduces your accuracy by 20%:
http://www.ufopaedia.org/index.php?title=Accuracy_formula
Solution 2:
Thats because grenades in X-COM are incredibly effective, some play with them as their main weapon with a rifle (of some sort) being the backup weapon.