Not sure what you mean by 'NRPN 9'. You need to use the TWO controllers for NRPN, and these are CC 98 and 99 (decimal). Controller 98 contains the LSB, while 99 contains the MSB. So you are correct, you need to use the two bytes of data, and this will allow you to enter a larger value.
These things have been referred to earlier in this thread.
Another problem may relate to what you're sending data to, and what data you send. Are you wanting to send data to alter the EnvD setting directly, which may require data in one format. Or do you want to alter the parameters/operation of the EnvD knob, which may need something different. You should check the manual to see which should be the best way to do it.