Barring any surprises, the first Stone Thanos should have obtained was the Mind Stone within the Mind Scepter. But the Mind Stone had difficulty fully unleashing his power. Especially against figures like Odin, Thanos had no way to use the Mind Stone to secure victory. That was why he cooperated with Loki and sent him to Earth to seize the Space Stone.
With the Mind Stone and Space Stone working together, he should have been able to control countless living beings regardless of distance. At that point, Thanos could even command innumerable lives across the universe, forming a cosmic army to attack any force that possessed Infinity Stones and collect all six Infinity Stones.
But unfortunately, Thanos's plan failed outright, whether in the original timeline or in the timeline where He-El existed now. Even the Mind Scepter ended up stranded on Earth.
The second Stone Thanos obtained should actually have been the Reality Stone.
To prevent two Stones from being in Asgard at once, and to avoid Asgard falling and someone obtaining two Stones in one stroke, Asgard sent the Reality Stone to The Collector for safekeeping.
The Collector clearly knew the story of the Infinity Stones. Even so, he was killed by Thanos, who seized the Reality Stone from him before attacking the Nova Corps.
Or perhaps, while the Reality Stone was still in Asgard, Thanos had already glimpsed the changes brought about by Asgard's war. Once he learned that the Reality Stone had fallen into Asgard's hands and that Asgard intended to send it to The Collector, Thanos hurriedly killed The Collector, disguised himself as him, received the Reality Stone delivered by Asgard, and only then attacked the Nova Corps to seize the Power Stone.
Or perhaps Thanos knew nothing of Asgard's situation. He simply killed The Collector