With respect to the output, yes, it is fully floating and completely isolated from the mains system. You are free to earth ground either output terminal or it may be left floating.
There is, of course, a float voltage limit that is listed in the data sheet. Refer to http://literature.cdn.keysight.com/litweb/pdf/5991-4194EN.pdf?id=2445976 for up to date information.
Important: You cannot put units in series or parallel - even for a DC voltage.
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