Explicitly? No. Here is the list of behaviours that are banned in FFXIV. There is no specific reference to multiboxing, or using multiple clients (other than a prohibition against using multiple accounts for rank manipulation).
If Square Enix didn't like what they see and wanted to? Yes. There is a prohibition against "Creating, distributing, using, or promoting utilities that interact with the game". It's worth mentioning, however, that there has been similar language in the Blizzard EULA for WoW since forever, all the time we have been playing it, so the existence of such a clause is not necessarily a de facto ban on multi-boxing using ISBoxer or whatever.
It's also worth mentioning that this clause is often invoked in the FFXIV community when discussing ACT and other combat log parsers. ACT is used by a pretty significant portion of the community, and the only time I can remember that anyone has ever been actioned for using it was when a bunch of Japanese world-first raiders used ACT data to bully and harass other players. (This incident is literally what brought about the game's current anti-harassment policy).
Under this precedent, particularly as multi-boxing is just not a thing you ever really see in FFXIV, I would assume the principle of "if you aren't being a jerk to anyone or impeding anybody's play, we'll keep taking your money" will hold.
As Andreauk says, though, you may want to exercise caution pushing your money onto the table.
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