OK, I had a look at this I couldn't get the 3 position switch option to work but I could set a throttle hold for each turbine using two 2 position switches, I'll describe how this is done with photos below
1) In the define mixer screen set up two new mixers with the throttle and fixed value as an input, select a separate switch for each mixer to change between throttle control and fixed value. Here I called the mixers throttle-1 and throttle-2 and used switches 1 and 5 to change between the settings.
Then assign these to the relevant turbine throttle channels in the servo assign menu, here I used channels 8 and 9
Then in the mixer setting menu set the throttle input to be 100% and the fixed value to be the throttle hold value, here I used -80% as travel goes from -100 to +100%
Now each switch changes the throttle control from the throttle stick to the fixed value, so to to start the turbines, you'd start with both switches in the fixed value position, then activate one switch the stick now controls that turbine, start the turbine. Once started put the switch into the fixed value, adjusting the fixed value to give you your desired turbine speed. Move the other switch to activate throttle control on the other turbine and start up, once started move the other turbine from fixed value to throttle control and the throttle stick now controls them both and the throttle cut if set will send the throttles to fully closed in the throttle stick active mode.
Hope this helps, if you have any other preference, e.g. using the central sliders to start each turbine, let me know.
Sorry about the picture quality, taken on my phone in my workshop.
