Air carrier pilot reported while on short final a twin piston aircraft taxied onto the runway; resulting in executing a missed approach to avoid collision.

Narrative

While descending into MYNN; we checked in with Approach Control and was cleared to a point on the RNAV 32 approach and told to expect RWY 32. The weather was clear and the winds were reported on the ATIS to be at 280@6 knots. Traffic landing RWY 32 and departing traffic RWY 28. We began the approach and were configured well above 1500 AGL with the checklist completed. We were trying to get landing clearance but with the traffic departing RWY 28; the controller was constantly talking and we waited for a pause. The Tower Controller was heavily engaged in a conversation with a twin piston concerning his position at the holdshort line for RWY 28 as to whether he was over the holdshort line. The pilot reported he was not over the line and the Tower Controller disagreed with the pilot but then told him LINE UP AND WAIT RWY 28; HOLD SHORT RWY 32. The pilot of the twin piston then read back to the controller; "LINE UP AND WAIT RWY 32". Since this was the first opportunity that the First Officer had to communicate with the tower controller he asked if we were cleared to land finally at 300 AGL. The Tower Controller verified that we were cleared to land RWY 32. I then asked the First Officer if he had heard the twin piston pilot state that he was gonna line up and wait RWY 32 and he stated that he didn't hear that because he was trying to get landing clearance. I told him to watch that aircraft just in case he had read that back. As we crossed the threshold at 50' I noticed the twin piston still moving towards the hold short line for RWY 32 on RWY 28 and as we passed 20' and I started to flare and retard the throttles to idle; the twin piston pulled out onto RWY 32 right in front of our aircraft. He was approximately 500 to 600 feet in front of us and I executed a very low wave off only a couple feet off the deck. I rapidly push the throttles up and called for the gear to come up because I felt that we were so close to colliding with the aircraft that it was the best option to avoid contact with the aircraft on the runway. After reviewing flight data; it appears that as we passed the aircraft we may have missed it by approximately 50 feet or less. We executed the missed approach and then flew another RNAV32 approach without incident. I spoke with the Tower Supervisor and he advised me that he had not heard the twin piston pilot read back the wrong instruction to him.Tower controller over whelmed with the dual RWY operations"