libanthy1t64 was released to experimental (and still is there) with the soname change.
After the package rename the Breaks/Replaces to make upgrades work seems to however have been missed.

    Unpacking libanthy2:amd64 (1:1.0.0.20260213-1~exp2)…
    dpkg: error processing archive /var/cache/apt/archives/libanthy2_1%3a1.0.0.20260213-1~exp2_amd64.deb (--unpack):
     trying to overwrite '/usr/lib/x86_64-linux-gnu/libanthy.so.0.1.0', which is also in package libanthy1t64:amd64 (1:1.0.0.20260213-1~exp1)

Hello,

Thanks for reporting this. This will be fixed in the next upload where
src:anthy will be uploaded to unstable soon, then the versions in
experimental will be gone.

Cheers,