No it can't
One major flaw you're forgetting, is the simple fact that the game lacks the stability across clients to do this. Technically, yes, it should be possible. But realistically. let me take your examples:
Dbowers: an account could lag, miss the target click, and then be still following button clicks, lag, tick times
Mining: Lag again, different tick times, other players coming to fuck you over
WC'ing: Lag again, different tick times, other players choping down the tree you clicked on one account that isn't available on another
Fming: Same as before + other players + missed ticks
Agility: One account lags, they're out of sync. An account fails to go over an obstacle, you gotta stop everything to fix it
These are just some examples of what you're saying is possible(which they're clearly not). There is a reason why we are not multiboxing and actually creating scripts. It's people like sirpuggers who introduce a fake idea to his audience, only for the fickle and retarded audience to follow and come here claiming they know more about botting on runescape than the people who earn a real life wage, and possibly more, from botting on runescape.
tl;dr: If you think you can use multiboxing to do anything other than bank standing, you need to grow some common sense and stop watching sir puggers