I'm not familiar with that set- was never available here in the UK.
I know there are others on here with that set, or who have used them, but the cat seems to have got their tongues, so I've had a gander at the manual. The mixing looks to be comprehensive and flexible. You have a 'Master and 'Slave' arrangement like on most mixers (all sounds a bit kinky really). So for mixing throttles you want the channel normally used as a rudder set up as master, and set the rate to 100%. Then you have the channel normally used for throttle, that should be set as slave, and try setting the rate to 50%. Now with one ESC plugged into either channel, with sticks at neutral I think the motors should be off, as you bring the throttle up, both motors should come on at equal rate, move the rudder stick to one side or the other, and the throttle should bias to one side or the other.
Now that's my best guess, and it might be totally wrong- without a set in front of me it's difficult, but you can't do any harm playing about with this sort of thing, and it only takes seconds to set it up.
Really flexible throttle mixing can be done with an onboard mixer custom designed for the job, but you haven't really room for that kind of thing in that micro sub unless you can program your own onto a SMD microcontroller.