The way a 2 cylinder 2 stroke crankcase is engineered
each cylinder is fed by it's own intake chamber, and the flow
to each chamber is controlled by it's reed valves. If a reed is sticking
it interferes with the amount of fuel/air mix entering the intake side of that cylinder.
An air leak into that chamber will cause the same problem, possibly a reed block gasket split.
If there is a coolant leak due to a failing gasket or a hairline crack in the block
that small amount of water will vaporize in the intake chamber due to engine heat
and change the amount of fuel/air entering the firing chamber.
Take a look at the linked diagram, and remember you have 2 cylinders.
One cylinder is working as expected, the other has a problem.
http://science.howstuffworks.com/transport/engines-equipment/two-stroke6.htm