When equipping an weapon on a Mecha, the message 'BACKPACK_Weapon_Need2Hands': "This weapon is too heavy for you to use in just one hand" can be generated without the affecting the player's ability to equip the weapon onehanded.
From: EquipItemFrontend in backpack.pp
Logged In: YES
user_id=1753058
Originator: NO
That's the intended behavior. Any weapon can be used in one hand, but will suffer a serious penalty to attack rolls if it's too big. The message has been changed to hopefully make this more clear.
Logged In: YES
user_id=1907337
Originator: YES
So the penalty is inflicted as long at the second hand is occupied?
Logged In: YES
user_id=1753058
Originator: NO
Yes, that's right.