Testudo and Synaspismos

[D B Campbell]

I'm just wondering if in this pace, by "synaspismos", Arrian means the distance of 1,5 foot for each soldier, as in a phalanx, or if he simply used the word in its litteral meaning of "shield touching each other" ...

I would say it's the latter.

Arrian doesn't actually define the spacing of men in synaspismos formation (afaik), unlike Asclepiodotus and Aelian, who draw a distinction (for the Macedonian phalanx) between araios ("loose" order) of 4 cubits and pyknos ("close" order) of 1 cubit, and who equate the latter with synaspismos ("holding shields together").