GS500 engine is a parallel twin engine.....thats what most of us know........but what most of us dont know is that it's crank angle is 270 degrees. which means one piston follows three quarters of a rotation behind the other. This yields firing intervals identical to a 90? V-twin. Firing order here is that cylinder 2 fires 270 degrees (3/4 of a rotation) after cylinder 1, and cylinder 1 fires again 450 degrees (one and a quarter rotations) after cylinder two, again at a total 720 degrees and the beginning of the next cycle.
Whenever we think of a parallel twin engine we thinks of the 180 degree crank angle matlab when one piston rises the other one falls. In this configuration the Pimary and secondary imbalances are minimal. Same is the case with 90 degree V twin engine. However some engine configurations have inherent imbalances such as the 270 degree crankshaft angle of a parallel twin. The reason why manufacturers use a 270 degree crank angle is because it imitates and sound and feel of a 90 degree V twin (which is costlier to built).
Obviously when you shut down one of the two cylinders in a parallel twin motor then you have to balance the Crankshaft. There are 2 types of balances, primary and secondary balances, in this given case the secondary balance needs to be achieved to compensate for the non existing reciprocating force of the shutdown piston.
In theory the secondary balance cannot to achieved in any case whereas primary balance can be achieved 100% by adding or removing weight from the crankshaft. Increase in secondary imbalance of the engine will increase vibrations but the stress levels will remain the same.