In the old days, with a EC135T1 and electromechanical instruments, you could select both engines to flight straight away, and as soon as practical go and fly.
On the EFIS /CPDS machines, the pilots flying display (PFD) and nav. display (ND) must be up and running before flight is selected on the engine select switches. It only takes a few seconds and does not present any real delay to the flight.