Caution on one thing: both KUL and SIN operate budget terminals. In SIN it's not that big a deal - there's a shuttle bus that operates from the main terminal buildings to the Budget Terminal. However the KUL Budget terminal is far away from the main terminals at KUL (essentially the budget terminal is on the other side of the runways from the main terminal). It's a bit less than ten minutes away by taxi, or I guess 15 minutes by local bus, fare 1 Malaysian ringgit (say 35 cents US).

Be sure to check who flies into where, especially if you book any of the budget carriers (Air Asia, Tiger, Jetstar) on this routing but need to transit to a legacy carrier; 3Ten above gives good info on this. Also assume that there's no interlining of baggage, so you won't be able to check bags all the way through. Last but not least know that if you need to transit at either location, you will need to clear immigration and customs on arrival, then check in at the other counter and go through immigration and customs formalities. This can make for tight connections. In other words if say you were flying Air Asia SIN/KUL, you would touch down at the KL Budget Terminal and legally enter Malaysia. If you were using KUL as a transit to a CX flight to HKG, then you would have to take a taxi to get to KUL Main terminal, check in with CX, then legally exit Malaysia by going through Immigration again.

edsh also makes a good suggestion for taking Firefly - it's a Malaysia airlines subsidiary that flies turboprop aircraft. However Subang International is a lot closer to Kuala Lumpur that KLIA (Subang used to be KUL). But try not to get into a situation where you have to go from Subang to KLIA (in the town of Supang). They're easily 70 miles apart!

Finally, it should be possible to construct one itinerary from PDX to SIN and KUL. You could do with it with some combination of UA and SQ (but not MI, SilkAir, a SQ subsidiary). But if you're trying to accumulate or use oneworld miles, then a 100% oneworld itinerary is not possible AFAIK.