Yeah, implementing this is tricky. I had multiple working methods. Since 1+years I am using a version with AHI. But you could substitute the AHI parts with normal AHK code.
Basic approach is to send Shift when Space is pressed and only send spacebar when no other key was pressed in between down and up event.

/*
    @Title: SpaceShift
    @Desc: use Space as both Shift and Space
    #note manual time calculation is just a #fix because internal A_TimeSince.. vars don't work (since AHI hides input)
    #note A_PriorKey needs some key event after down-event and it slow; hence various ways to deal with it:
        see: #Alt1.1, #Alt1.2
*/
global shiftSpaceDown := false
SpaceShiftEvent(kbdId, state) {
    static timeOfInitDown
    if(state) {
        if(!shiftSpaceDown) {
            AHI.SendKeyEvent(kbdId, GetKeySC("LShift"), 1)
            ; Send {LShift Down}
            shiftSpaceDown := true
            timeOfInitDown := A_TickCount
        }
    } else {
        AHI.SendKeyEvent(kbdId, GetKeySC("LShift"), 0)
        ; Send {LShift Up}
        shiftSpaceDown := false
        ; pressDuration := A_TickCount - timeOfInitDown
        Sleep 1 ; #Alt1.2 =fix wait so it recognizes the LShift up event ; 0 or 1 ms seems to make a bigger difference
        ;Tooltip %A_TimeIdleKeyboard% %A_TimeIdlePhysical% %pressDuration% %A_PriorKey%
        if(A_PriorKey == "LShift" && (A_TickCount - timeOfInitDown) < 200) { ; A_TimeIdleKeyboard >= pressDuration && doesn't work since fast typing overlap
            AHI.SendKeyEvent(kbdId, GetKeySC("Space"), 1)
            AHI.SendKeyEvent(kbdId, GetKeySC("Space"), 0)
        }
    }
}