The Ultimate Middle-Aged Hunter Travels to Another World ~This Time, He Wants to Live a Slow and Peaceful Life~ Other than that, I remember part of Heroes Union BBS story 'bout a guy called Freeter who got Isekai-ed so many times Sometimes right after he just finished one world, he'll got transported again to another
Spoiler I dont think the op wants something like it because although it's double isekai the previous world the MC was in and the world before it were modern world and he uses modern knowledge he knew in the current world he resides in.
there is Warlock of the Magus World the mc comes from an earth-like world and they don't talk about it much except on the last chapter and first few. he also travels to many different worlds and dimensions. and with the assistance of his a.i. chip makes many spells and skills
In Arifureta end stories, some of the cast gets isekaied again. One got double isekai. Demon noble girl has a quad isekai, mc has vague memories before being a demon, gets born as a demon in a human, and then gets involved in a summoning ritual, and now current chapter has her on a spaceship. FFF Class Trash Hero has mc run simulation isekai worlds The novels extra really enjoys isekai, has the mc travel to another world while in another world before returning
Just remembered these titles, not sure if this count as "isekai to isekai": I’m Back in the Other World? -> a wizard (male) of a fantasy world died, reincarnated & gender bend into modern normal world w/ intact memory, but got summoned by the royals of the first/previous world Will the Re-Summoned Hero Live as an Ordinary Person -> former hero. After finishing his deed, got sent back to normal world. But somehow got entangled into a summoning circle & back he goes to the fantasy world that he left