This was brought up on the talk page of the History of the Pokemon World page, and I'd like to ask about it.
What solid evidence is there that Generations 1 and 3 take place at the same time, and 2 and 4 take place at the same time? As said on the talk page, BW have shown that Platinum is the canon version for the Sinnoh games, where the Red Gyrados report didn't happen. Steven is also still the champion in HGSS and only makes vague comments about similar challengers, not to the actual events of the Hoenn games. Remember in Emerald he actually decided to leave and travel, not intending on returning for a while. I find it hard to believe that Steven would come back after his long journey to become the Hoenn champion again, only to leave for Kanto. It's unlikely his appearance in HGSS is after RSE. Also in HGSS, Cynthia doesn't mention the events of DPPt at all, so whether she's champion or not in Generation 2 is unknown and could very easily take place before Generation 4. The hiker in Hearthrome city mentions Elm's report on eggs, despite the fact that FRLG showed that pokemon eggs were around before gen 2. That simply means the hiker remembers the report from a few years ago and that Elm was the one who did the most research on the subject, as seen in HGSS. Jasmine's conversation with Erika in HGSS shows plainly that she didn't go to Sinnoh until after HGSS. And on the subject of Jasmine, before somebody brings up Rusty the Steelix, the idea of her having two pokemon with the same nickname isn't really that far-fetched. And if Jasmine traded Rusty in HGSS, then how could it be the same Rusty that appeared in contests with her in DPPt?
It seems the main reason for the 'Gen 1 and Gen 3 take place at the same time' and 'Gen 2 and Gen 4 take place at the same time' is, as said on the mentioned talk page, trading without time travel. As said, this an extrememly flimsy justification. XD and Colloseum take place five years apart, yet both can trade with the Generation 3 games. The upcoming Black 2 and White 2 games will also likely be able to trade with the original Black and White.
Just thought I'd put this out there.