You will be fine with that scenario. Chill.
The 600W transformer is actually 2x300W cores. So if you were to load one side to say 260W and the other had only 60W, you would probably see a discrepancy in secondary output voltages between the two sides. (most transformers 'drop volts' when they are loaded up to / near capacity) If you are 240w on one and 200w on the other there may be some small discrepancy, but nothing you should really worry too much about.
Of course, if you were to use LED lamps, you would not need such a large transformer and never would have this issue.