I got Nalis' ending on my first playthrough and then used the guide to get Thaumo's endings and I don't even know how I got Nalis the first time because all of my choices leaned towards Thaumo. I think the only choice I made between the two that got me Nalis' ending was questioning Thaumo about the spilled drink. To get Thaumo's ending instead, I only changed that to thanking him, otherwise I got the Nalis ending.