Longest MCT for SQ-SQ on International to International is 90 minutes, so your two hours are fine and transfer in SIN is fast. The only problem can arise if your inbound flight is delayed. KUL-SIN has generally reasonable on-time performance stats, but I would check the exact flight you plan to take. There seems to be heaps of flights, so on two PNR's it is always better to have longer wait at the airport than be sorry...