It depends what are looking for during the your stay in KL & S'pore. basically you can visit all the attractions within KL in a day time. If you would like to shop, then another day with all the nice food of course.
1 day will spend of your journey from KUL - S'pore if by bus - appx 6 hours.
Singapore if u going to USS - 1 day gone.... sightseeing about S'pore - another day gone.... if buy branded...buying at S'pore is cheaper than KUL. Not sure what time is ur flight...just giving you the idea of it... Hope it helps.
You can get night bus from KUL-S'pore so you save 1 night accommodation & more time to spend in S'pore.
Day 1 Arrival
Day 2 KUL sightseeing
Day 3 Shopping
Day 4 KUL - S'pore
Day 5 S'pore sightseeing
Day 6 Departure almost 6 years ago
I have some suggestions here if you are looking for a unique and authentic Malay experience;
http://answers.worldnomads.com/questions/malaysia/19286/best-way-to-sightsee-kl-while-in-malaysia-travel-agency almost 6 years ago