You are trying to short out the first transistor base with the opto-isolator.
That's not a good design as it means the switching point is around 0.6V
Normally, the opto transistor should be between the base and +12V, with a series resistor to set the "on" current to the base, plus a base-emitter resistor on the transistor to ensure turn-off.
Unfortunately the MCT2E opto is a very "slow" device, according to the datasheet it can take up to 50uS to turn off. That's an entire half cycle at 10KHz.
You need an opto isolator that takes eg. no more than 1% of your cycle time, for optimum efficiency.
See page 4 of the datasheet here:
**broken link removed**
Adding a resistor between the base and emitter connections of the opto transistor will speed it up a bit but also decrease it's sensitivity.
With bipolar devices you can speed up the turn-off time somewhat by adding a schottky diode between collector and base, with anode to base & cathode to collector.
That prevents full saturation as the diode takes the base drive away when the collector gets down to 0.2 - 0.3V and can make the turn-off time quite a lot faster in switching applications.
You are making a "Schottky transistor" with discrete components:
http://www.mikrocontroller.net/attachment/14537/pullr_6.gif
It can help with optos that have the base available on a pin, as well as general transistors like T1 in your schematic.
Both that and the base resistor on the opto may each give some improvement, though a much faster opto is really needed.