I have a toughness 1 creature. My opponent has two creatures, each with "Cloud of Flies" attached to it. At the start of his turn, he uses both Cloud of flies on my toughness 1 unit. Does one damage get canceled or both? I think it's important if one assigned damage is immediately resolved or both damage get assigned / resolved all together.

You should read clood of flies egain :

Card text is :

Attach to a target unit you control. At the beginning of your turn, you may deal 1 uncancellable damage to this unit and to one target unit.

Clood of flies damage are uncancellable you can not use thougtness on it. (as thoughtness cancel damage).

If those damage were not uncancellable your 1 thoughtness unit would have cancel both damages, as damage are made 1 by 1.

Thanks for clearing that for me. Also, how damage is done if it's not cancel-able.

And beware! ...as I have some more questions coming

Krabat said:

Not sure I understand the question, but uncancellable dmg is just like any other damage, assigned and applied, just with no opportunity for cancellation effects in between.

Krabat said:

If we pretend that the damage from Cloud of Flies isn't uncancellable, you would activate each cloud of flies in turn, thus you would assign and deal 1 point of damage to the target which then gets cancelled by Toughness and then again, 1 point of damage which gets cancelled. You always resolve one action at a time.

You can however set up a chain of actions using responses, but they would still resolve one at a time.