The documentation says this about runToNewPosition(...):
"Moves the motor (with acceleration/deceleration) to the new target position and blocks until it is at position. "
The problem is the word "blocks". To move two motors virtually simultaneously, you will have to use one of the non-blocking functions (such as run()) in a loop.
You may want to look at the library source code for runToNewPosition(...) and do what it does but do it for two motors.