aha! you just answered your own question!
always 2 coat unless youre only in it for the money and u can definately get away with it and youll never see the site agent/customer again..

lay on your first coat, then when its gone a bit go over it with your biggest trowel, youll take 90 percent if not all of the lines/ridges/hollows out at this point
then when you mix up again, the setting time is different, gives you a better chance and a much better finish
any hollows etc will be filled in now (think 'first coat gets it flat, the second even flatter)...
if you top it with the same mix youll acheive a better finish than 1 coat but nowhere near as good as with 2nd coat new mix..
youll be surprised at how easy youll make it for yourself...
youll also be surprised at out of flat your wall was after the first coat when you top it...
tip - keep your trowel as flat as you can for as long as you can, if youre pulling plaster back off the wall in large amounts as you flatten/trowel it, your too steep...
keeping a shallow angle will flatten the wall for you so when you come to finish it the jobs easy...
takes practice keeping a shallow angle as youll have a tendency to go too shallow and stick the trowel to the wall, pulling of lumps of plaster when you remove it...
go slowly and deliberately