Hy LEM:
I think this procedure has no base,and also it's not safe.
If you'll check the air press during start,you'll notice that it fluctuates,not much though.
Also you'll have starter cutout at 46% ,but you must monitor the engine param until eng stabilised. What if,after eng2 starter cutout,while you're starting eng 1 (monitoring valve ,oil press,N2,N1,...) ,you'll have a rapid increase in temp at already started eng nr2 (but not yet stabilised)?Or no oil pressure at stabilised eng? Who is monitoring that?
It's a stupid thing to do,my guess.And you'll see this at the first engine start problem they'll have.
START VALVE OPEN light indicates valve is open and air is supplied to the starter.