You will feel when its ready mate.
Once the first coat is on, clean your hawk, trowel, buckets etc... Then take the lines out of the first coat with your trowel, starting where you started laying it on.
Clean your trowel and get set up for the next mix.
When your mix is ready, take any large lines out that are still left, and get your second coat on.
When you are taking the lines out of the first coat, you will notice the plaster is stiffening up a bit. If its not, then wait a little longer and try again.
Basically, your second coat goes onto the first, when the first coat is 'pulling in'.
Don't try to lay your second coat on when the first is still wet, or it kinda defeats the purpose of a second coat, as the first coat will move around underneath your trowel.
meaning if i can get across a 10ft wall in one coat and then its ready to flatten,would i get across 2 walls of 10ft if i layed onto them both then applied my second coats?
point being,would it give me time to do both walls if i used 2 coats?
i usually only use one "thick" coat then flatten in.Only put 2 coats on if its rough.
2nd coat is applied a lot thinner than the 1st coat, I also have mine a fair bit softer and a new gauge if possible. You will find that using 2 coats will improve your overall finish. Worked with a fair few spreads in my time and I have never seen one only use 1 coat.
When i was doing my courses l was told to put the 2nd coat on as soon as you've finished the first, instructors always said dont worry about flatting the first coat, as its all about speed get your coats on as quick as possible then go over it filling in any holes or dips, they said dont even worry about any small lines as these can be taken out while you polish up