Should you visit peninsular Malaysia and Borneo in one trip?
You can combine them, but Borneo needs real days. Treat Sabah or Sarawak as a proper second chapter, not a quick wildlife garnish after KL, Penang and the beach.
Answer
Peninsular Malaysia and Borneo can make a brilliant pairing, but only when the itinerary admits they are different trips. The peninsula is easier for city food, trains, buses, Grab, islands and shorter hops. Borneo is more about flights, wildlife, rainforest, mountains, caves and guided logistics. Add it casually and you risk spending too much of Malaysia in airports and vans. Add it deliberately and it can be the part that changes the whole trip. For a first visit under two weeks, choose either KL and Penang plus one beach, or KL plus a focused Sabah or Sarawak route. Borneo is worth the distance when it has enough room to breathe.