It is because Input.is_action_pressed("Shift")
is only pressed in your computer.
Try this
func takeInputs():
var speed_mul = 1
#code to take input
#get direction and all
#
if Input.is_action_pressed("Shift"):
speed_mul = 2
rpc("calculateVelocity",direction1, speed_mul)
sync func calculateVelocity(direction1, mul) -> void:
velocity = direction1 * speed
move_and_slide(velocity)