Holding 2 bits of string and a stick (more great tips here >>>)

The important part is to have a system that works for you and practice it so when you need to trim the sails you not thinking which rope is which or how to hold them.

I usually trim the kite sheet in my front hand and hook the main sheet over my thumb with a bit of slack so it doesn't affect steering and I am ready to gybe (below)

both sheets in the front hand you can trim both sails together from bent arm to straight arm, by holding one sheet in the thumb,palm and little finger and the other Thumb and index fingers you can let one sheet slip to fine tune the relationship between the main and kite. (below)

Both sheets in the tiller hand gives one hand free to adjust trapese hieght or select one sheet to adjust on its own. I would have the main hooked over my thumb and Kite sheet in my fingers this is used at the beginning of a gybe. at the right moment I release the old Kite sheet from my fingers as I pick up the new sheet, but keep the main sheet hooked over my thumb (below)

I treat the mainsheet and tiller extension as one when I change hands behind my back, the same for tacks and gybes (below)