EDIT; 3 years later: updated the script to no longer use the data from the spreadsheet, but rather from tornplayground.eu (developed by me) and even auto update that data using the script. The website also looks at the spreadsheet for fresher prices.
The mods are only active during the turn you use the weapon.
So if you use the arma on turn 1 you'll have the efffect of the tactical illuminator on turn 1, and using the secondary on turn 2 you'll have the effect of the precision light on turn 2