Originally posted by Springheel:

; Remap Left Shift to Right Mouse button and Space
LShift::
Send, {RButton}
Send, {Space}
return

Have you tried switching the order of the inputs? I do know that they are not sent at the same time, on a technical level. That might have something to do with it.

Also might and to try adding "down" and "up" indications. See the following reddit thread for examples.

https://www.reddit.com/r/AutoHotkey/comments/3yi6w1/two_keys_at_once/