When charged objects touch, you can assume that the charges move between the objects, so that the total amount of charge doesn't change but it splits equally between the two objects.
-- like two water tanks standing next to each other, with a different amount of water in each one. When you connect a pipe between their bottoms, some water flows across until the LEVEL of water is the same in both tanks.
-- like one hard full balloon and one soft mooshy balloon. When you connect them together, some air flows from the hard balloon into the soft balloon, until the pressure of air is the same in both balloons.
The total amount of charge on your two objects is (+6.0 μC - 2.0 μC). That's +4.0 μC .
When they touch, charges move around until the charge is the same on both objects . . . +2 μC.