My question is whether it is legal to say, take swords and shields in the first rank and halberds in the second rank of a squad of Chaos Warriors.
If your guys can actually purchase more than one special weapon (eg. If they have the option to take both a halberd and a Great Weapon), you can carry as many as you want. Note that not many units even get the option to take more than one weapon, so double check the unit entry to see if they can buy more than one. Units get to choose at the beginning of each combat which weapon the unit as a whole will use. You must then use that weapon for the rest of the combat, and cannot change.now can you pay the points for different weapons and use them at different times?
For completeness, they are only forced to use the special weapon in close combat, so can still gain an increased save from a shield against shooting attacks.... if you have a special weapon you have to use it, so if you get halberds or great weapons for your Warriors, they can't use the Hand-weapon/shield option at all.