Based on all my research into this event, there was in fact a 2nd shooter which was positioned on the roof of AMPM ~300 yards North of the fairgrounds stage (shooter #1 from the Mandalay Bay 32nd floor ~500 yards South-West of the fairgrounds targeted the Southern fairgrounds and the people fleeing South, while shooter #2 from the AMPM roof ~250 Yards North of the fairgrounds targeted the Northern fairgrounds and the people fleeing North).
Bird's-Eye View of Both Shooting Positions (Primary Attack + Secondary Ambush)
Angled View of 2nd Shooter Position (Secondary Ambush)
32nd Floor Perspective View of 1st Shooter Nest Position (Primary Attack)
sidenote: All text in "quotes" below are from police scanner transmissions from that night (over a ~60 minute period).
Shooter #2 not only targeted the fairgrounds but also targeted people East of the Catholic shrine by Giles Street and Ali Baba NE of the fairgrounds ("Control 224-IC confirming I have casualties at Ali Baba and Giles east of the Catholic shrine as well as in the Mandalay Bay, so we have 2 scenes") as well as targeting medics responding to the scene who were positions over near the street by the entrance to the Tropicana ("Control 374 I got medics reporting they are being shot at, at the Tropicana"... "Be advised that there is an active shooter at the Tropicana, active shooter Tropicana"), who after the fairground shooting fled North through the western Tropicana parking lot, before crossing the street by the Excalibur and then entering New York New York, but upon entering New York New York is IDed by the front desk as an active shooter ("We're getting a 415A, from the advisor that there was a shooter at the front desk [New York New York].") causing him to flee the scene, but before getting out of the building is confronted which causes him to shoot his way out ("Control 35-SD we have possible reports of shots fired inside New York New York"), fleeing South back the way he came towards Excalibur ("They have records at the 415A at New York there might of been an active shooter possibly coming down the escalator from New York New York to Excalibur") before crossing the road and getting in a white RV parked back over in the western Tropicana parking lot ("There is 1 WMA [White Male Adult] in black fatigues in a white RV off of Cobalt and Tropicana") where he again attempted to flee the scene North but is stopped near the intersection by the Bellagio and Caesars Palace where he gets in a standoff with police which leads to a shootout ("Be advised that now we are getting shots fired at Caesars and the Bellagio").
The entire 2nd shooter event from first shots fired (at fairground) to last shots fired (by the Bellagio) took place over approximately 1 hour period of time . As added context the shooting by the Bellagio took place ~45 minutes after the fairground shooting had already finished.
Sidenote: It is also possible that the 2nd shooter was instead on the raised billboard a few feet West of the AMPM. Both positions are in extremely close proximity to each other, offer a slightly elevated vantage point of the fairground, provide full ambush coverage of the Northern escape path, are within distance of the fairground the Catholic Shrine and the Tropicana (all 3 of which are shot at), and allow for a clear Northern escape path for the shooter to run in the aftermath (basically think of both these 2 positions as interchangeable).
Street View of Potential 2nd Shooter Alternate Ambush Position (10 Feet West of AMPM)
Added Context: Forensic acoustic proof of SECOND shooter in the Las Vegas massacre