I had the funny idea that once the OG Regi's meet Regidrago and Regieleki, they are gonna question Regigigas about why she never told them about these two other Regi's.
And Regigigas, in all of her sleepy wisdom, woukd simply say "Oh, I suppose I forgot to tell you about them, teehee"
You probably already have an idea for how the two Galar Regi's will be introduced, but I think this is a funny option if you can't think of good interactions.