Reading transformer datasheet
If secondaries have the same output voltage (i.e. number of turns) you can either parallel them (for twice current/half output Z - which boils down to the same thing) or put them in series (twice the voltage, twice the output Z). 

The only thing is to take care of phase - if you parallel them so that the phases are fighting each other, they are likely to get hot (on a power transformer) and it might get nasty. (I've seen this as a wiring error on the secondary of a small mains transformer and it did some quite odd things, but no damage occurred. On something with more VA I would not be so sure.)

Your third example - the note about tying the terminals implies that the manufacturer expects it to be used with paralleled  secondaries (as that is making connections as described would do).

The voltage/current ratings are probably the maximum rated for each one, which I expect that that would be somewhat short of saturation, as any coil becomes a lot less efficient when saturated.