Hello,
In original Xcom, when a soldier uses 2 weapons, the player can choose which weapon is active.
Say, you have a soldier with a Rocket Launcher (Right Hand slot) and a Laser Pistol (Left Hand slot).
During combat mission, the soldier selected, click on the Laser Pistol (Use Left Hand Item) with the left mouse button, then click outside the menu (right or left mouse button in original Xcom, left mouse button in OpenXcom) to escape the menu (Aimed Shot - Snap Shot - Auto Shot - Throw).
Now, the Laser Pistol is the active weapon, and if the "reserve TUs for Snap Shot" button is "on", then the needed TUs to use the Laser Pistol are reserved (ex: 12 TU).
And if the soldier need to fire (reaction fire) during enemy turn, he will use the Laser Pistol.
If the player click (the soldier selected) on the Rocket Launcher (Use Right Hand Item) with the left mouse button, then click outside the menu to escape the menu; now, the Rocket Launcher is the active weapon, and if the "reserve TUs for Snap Shot" button is "on", then the needed TUs to use the Rocket Launcher are reserved (ex: 27 TU).
And if the soldier need to fire (reaction fire) during enemy turn, he will use the Rocket Launcher.
It does not work this way in OpenXcom.
Hope that helps ;-)