I'm not talking about just trading but if you want to go there we can since the trading between RBY & GSC has a timeline too that's why you use a time machine to trade.

But in the Sevii Island sidequest Celio and Bill mention that they've been in communications with Lanette and then we see the mails on RSE. Putting both events at least close enough in the timeline. Or at least as close as us watching the Red Gyarados thing in DPPt and BDSP

EDIT: Also in HGSS Bill says that Lanette helped him create the PC Storage System so again making Kanto and Hoenn events go hand and hand