I'd translate 同然 as "so similar that it can now be considered X" or "as if it is X". It states there is effectively no difference between the two. E.g. 夫婦も同然の二人、もはや勝ったも同然だ.
同様 would be more generally, "in the same manner to X", e.g. 炭酸と同様の作用を有する、操作は先ほどと同様です
... same tax rules are applied
... same tax rules are applied, such that there is essentially no difference between same-sex vs hetero-sex marriage.
彼のと同然の靴 would be weird as you would be stating "the shoe can be considered to be essentially his". When expressed more naturally, this will be