My interpretation: the 30VA rating is applicable only to the 'whole transformer' but with the proviso that 30 VA rating only applies to both windings of each type.
i.e. each 12V windings by itself is only good for 1/2 of the total VA rating, i.e. 15 VA / 12V = 1.25A each.
Put them in series (24V) and you can get 1.25A. Put them in parallel and you can get 2.5A (i.e. 1.25A per winding).
Ditto for the primaries - each primary is rated for 1/2 the total VA so 15 VA/115V or 0.13A
You can mix and match but you shouldn't exceed the current rating for any one winding. I also think you would want to keep an eye on core current (I am out on a limb here) but suppose you wanted to use one primary for the input and the other primary as an output along with both secondaries. I would probably be a bit conservative and split the 15VA rating of the input among the 3 outputs.
To more explicitly answer your question: if you parallel both primaries and drive with 115 VAC and parallel both 12V secondaries and use to drive your rectifiers then the output current rating (of the transformer) would be 12VAC at 2.5A. I think.
73
Bob Groh, WA2CKY