If your KUL connection requires going thru immigration you most likely will miss your flight.

2h connection time in SIN is more than enough. 2h connection time in KUL is barely enough if you have to go thru immigration and recheck luggage. When you have separate tickets, I would add one more hour for KUL.

As someone sugggested, book non-stop flight to your final destination if that is available and save yourself from misery.