Darn it! I knew this word would come up, and I just cannot spell simultaneously at all!
Worse than that: I have been pronouncing it 'simuNtaneously' all these years sobs
I'm debating mentally whether I should ask that 'together' be added... it's common to say together and mean simultaneously from experience, and simultaneous laughter does imply togetherness, but they are separate words for a reason...
A terribly late reply, but we have the same distinction in Swedish (samtidigt/tillsammans), so I don't think it should be added. They're not interchangeable enough to me.
In this case, att is actually optional, so it does work.
Edit one month later: And now I'm a moderator, so I've added it. :)