Yeah, for the most part the GA characters don't even acknowledge the existence of other characters from the same company, much less from a parallel world or alternate timeline!
That ain't necessarily so. Fawcett did a *lot* of guest cameos in their strips...a lot of them crossed over with Captain Marvel and he repaid the favor, often. (Don't forget Cap vs. Spy Smasher, either.) In MLJ, the Shield and the Wizard teamed up in an early four-parter, and their allies Roy and Dusty formed the Boy Buddies. In CAT-MAN COMICS, Kitten and the Mickey, the Deacon's kid, formed the Little Leaders with appearances from their mentors. Besides the Torch / Namor battles (which, in one issue, offered cameos from other Timely heroes), there was the All-Winners Squad, and the Young Allies, which not only starred Bucky and Toro but had guest-shots from the Torch and Captain America. Harvey regularly teamed their SPEED COMICS heroes in text stories, and put them all together in one Black Cat story against the Japanese. A batch of Quality heroes made cameos in one Uncle Sam story, and the Spider Widow and Phantom Lady had a string of crossovers. DC had two whole hero teams, the Justice Society of America and the Seven Soldiers of Victory. In BLUE BOLT, the Twister crossed over into every strip in the issue he debuted in. Uncle Sam summoned all the TARGET COMICS heroes together in one issue and gave them their assignments. I'm sure there are some I'm missing, but that's what I remember off the top of my head.