This is more or less "explained" in chapter 19, where Shadow talks to Whiskey Jack (quotes from the author's preferred text, emphasis mine):
“It’s not going to be a war.”
“Then what is it?”
Whiskey Jack crushed the beer can between his hands, pressing it until it was flat. “Look,” he said, and pointed to the waterfall. The sun was high enough that it caught the waterfall spray: a rainbow nimbus hung in the air. Shadow thought it was the most beautiful thing he had ever seen.
“It’s going to be a bloodbath,” said Whiskey Jack, flatly.
Shadow saw it then. He saw it all, stark in its simplicity. He shook his head, then he began to chuckle, and he shook his head some more, and the chuckle became a full-throated laugh.
“I’m fine,” said Shadow. “I just saw the hidden Indians. Not all of them. But I saw them anyhow.”
“Probably Ho Chunk, then. Those guys never could hide worth a damn.” He looked up at the sun. “Time to go back,” he said. He stood up.
“It’s a two-man con,” said Shadow. “It’s not a war at all, is it?”
Whiskey Jack patted Shadow’s arm. “You’re not so dumb,” he said.
And then when Shadow talks to Loki:
“I figured it out,” said Shadow. “Kind of. I’m not sure when I figured it out. Maybe when I was hanging on the tree. Maybe before. It was from something Wednesday said to me, at Christmas.”
Loki just stared at him, saying nothing.
“It’s just a two-man con,” said Shadow. “Like the bishop and the diamond necklace and the cop. Like the guy with the fiddle, and the guy who wants to buy the fiddle, and the poor sap in between them who pays for the fiddle. Two men, who appear to be on opposite sides, playing the same game.”
Loki whispered, “You are ridiculous.”
“Why? I liked what you did at the motel. That was smart. You needed to be there, to make sure that everything went according to plan. I saw you. I even realized who you were. And I still never twigged that you were their Mister World. Or maybe I did, somewhere down deep. I knew I knew your voice, anyway.”
Also the subject of this question.
Wednesday never told Shadow this plan, but he told him of the fiddle con and the jewellery con. The point of those is - two men playing on opposite sides are actually working together, to con some third party.
Up to that point Shadow knows a few things: there's Wednesday, there's Loki (who's supposedly working for the new gods), and there's going to be a bloodbath between the old gods and the new gods. He remembers the jewellery con, and he remembers Wednesday needs a partner for that:
“Hey. Wednesday. Both of the scams you were telling me about—the violin scam and the bishop one, the bishop and the cop—” He hesitated, trying to form his thought, to bring it into focus.
“What of them?”
Then he had it. “They’re both two-man scams. One guy on each side. Did you used to have a partner?” Shadow’s breath came in clouds. He promised himself that when he got to Lakeside he would spend some of his Christmas bonus on the warmest, thickest winter coat that money could buy.
“Yes,” said Wednesday. “Yes. I had a partner. A junior partner. But, alas, those days are gone.”
Wednesday had a junior partner. Someone who appeared to be working against him, but was in fact working with him. A very, very fitting candidate would be Loki, who's already working for the new gods. From there Shadow remembered that Loki's voice sounded a lot like Mr. World's voice, and from there he deduced the rest.
Wednesday is a god of war - he feeds on blood sacrifice. Loki is the good of chaos - he feeds on chaos. A "war", a bloodbath between the gods would provide nourishment for both.
If you're interested, there doesn't seem to be an "abridged" version of American Gods. Going by What was restored in the American Gods "tenth anniversary" edition?, nothing substantial was added to the book in the 10th anniversary edition, at least not enough to change such a major point.