#Requires AutoHotkey v2.0
ToggleMessageCount := 1
SetTimer ToggleMessage(ToggleMessageCount), 1800000

ToggleMessage(ToggleMessageCount)
{
    If (ToggleMessageCount / 2 = 0)
    {
        MsgBox "Do one thing"
    }
    Else
    {
        MsgBox "Do another thing"
    }
    ToggleMessageCount := ToggleMessageCount + 1
    Return ToggleMessageCount
}