How does off-hand spellcasting with two implements work?

With the Dual Implement Spellcaster feat, you can add the off-hand implement’s enhancement bonus to damage rolls.

1) What if you cast a spell with the off-hand implement? Do you get nothing from the main hand implement?

2) Is it even possible to cast a spell through the off-hand implement or do you have to designate one of your hands as your main hand and hold the implement you use in that one?