Just to point out, but gen 2 takes place 3 years after gen 1, so gen 6 (XY) can't take place during both those gens. Also, XY takes place after B2W2, which takes place 2 years after BW, which takes place some years after DPP/gen 2.
Unless you're referring to "ORAS gen 6," in which case, yes, it likely does take place during gen 1.
Which is why I say that having any sort of cameo is quite unlikely, unless Game Freak is willing to redesign their older characters to be young for these games, but that seems like a bit much to go through for just a simple cameo. It would also not match up with their own individual plots (Jellicent♀ brings up the excellent point that at this point in time N is not even King of Team Plasma--and is likely a young child--so there is no reason for him to be on his soul-searching journey--if anything, he is trapped inside his room being tortured by Ghetsis...and Ghetsis is not exploring the world, he is in Unova torturing N). If anyone has a good chance of having a cameo, it's likely Cynthia, who would only be 2(-ish) years younger than she was in DPP--honestly, DPP/HGSS and FRLG cameos are the only ones I can really see happening, because of their close proximity in the timeline. But anything past that I find close to impossible. Pokémon characters are usually younger than we think, so having them show in such early games (timeline-wise) seems highly unlikely to me. It would be cool, but then you'd realize it just doesn't make any sense and it would lose its appeal.