refers to the same action as the merger

The commas are wrong in the original. Should be, “adoptando, en el mismo acto de fusión, la denominación social de ésta última que...” or no commas at all. Nonetheless, I would translate as follows:
...con la sociedad XXXX, adoptando en el mismo acto de fusión, la denominación social de ésta última...
“with Company XXXX, adopting its name in said merger....”

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Note added at 2 hrs (2013-07-15 14:21:21 GMT)
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Or “in the aforementioned merger.”

There's no need to repeat merger as the Spanish and the previous three answers do. You can just say "The merger ... with X, in which it adopted the latter's name."