I do not understand what this question means: "Explain what most readers surmise Alexie proposes by juxtaposing Victor and Thomas."

Most readers surmise from the way in which Alexie juxtaposes Victor and Thomas that Victor and Thomas are two halves of the same whole and represent what the other needs. In helping Victor get to Phoenix to make arrangements for Victor's father's funeral, Thomas is literally the means by which Victor can do what he needs to do and achieve closure after his father's death. Thomas pays for the trip to Phoenix, and Victor, after claiming his father's savings account, pays for the trip home, symbolizing the way in which they are two halves of a whole.
In addition, Thomas provides the dreams and stories that Victor, a less spiritual soul, needs. For example, when Victor is young, Thomas tells him the story of a modern-day Indian warrior who steals a car and delivers it to the police station. Victor says, "That's a good one. I wish I could be a warrior." Thomas's stories inspire Victor. Later, Thomas says of himself:

"I have no brothers or sisters. I have only my stories which came to me before I even had the words to speak. I learned a thousand stories before I took my first thousand steps. They are all I have. It's all I can do."

Now that Victor has lost his father, he will also need these stories for sustenance, and he needs Thomas for spiritual guidance.
Later, Thomas tells Victor a story about Victor's father. Thomas had gone to Spokane to wait for a vision when Victor's father appeared and told him to go home before he got mugged. Thomas says, "Your dad was my vision. Take care of each other is what my dreams were saying. Take care of each other." By telling this story, Thomas reminds Victor about the caring parts of Victor's father, which Victor needs to hear. The story is about the importance of people looking out for each other, which is what Thomas does for Victor. At the end of the story, Victor gives Thomas half his father's body, as they share memories of Victor's father. When he gets home, Victor "heard a new story come to him in the silence afterwards." In other words, Victor has been inspired by Thomas to hear stories and think about life in more spiritual terms. The realist and the idealist have become one.