Is there anything halachically wrong for one to sell his chometz through two different Rabbis.
(it comes out that two different shluchim are selling the same chometz)?
There is nothing halachically wrong with doing this. Whichever Rov ended up selling the chometz first was the one who actually sold it. I remember seeing Horav Zaidel Epstein zt”l sell his chometz to two different Rabbonim.