The game doesn't allow exactly what you're asking for, but you can set up one key to turn both of them on, and another to turn them both off.
bind <key> +down$$-down$$powexec_toggleon Sprint$$powexec_toggleon Ninja Run
bind <key> powexec_toggleoff Sprint$$powexec_toggleoff Ninja Run
The only thing to keep in mind is that when pressing the key to activate them both, hold the key down until the first power (Ninja Run in this case, they're activated from last to first) fully activates. Then when you release it, the next power will start up.