Also, is there a definitive asnwer out there about balanced halfing the VA rating?
Let me take a stab at it again....
First a power transformer is designed to operate at its rated nominal voltage and frequency. The size of wire used is directly proportional to the power the xfrmr is capable of delivering.
The xfrmr you are looking to buy has dual primary windings and dual secondary windings.
For the rated output voltage/power of the xfmr the primary can be wired for 120V in (paralleled) or 240V in (series)
The FLA (full load amps) rating for the primary would be 15 amps @ 120V (1800Va / 120V = 15 amps) or 7.5 amps @ 240V (1800Va / 240V = 7.5 amps). Output of xfmr fully loaded...
For the rated output FLA of the secondary of the xfmr the xfrm can be wired for 120/240V out (series the two windings, midpoint becomes the neutral) or straight single ended 120V out (parallel the two secondary windings)
The 120/240V configuration is capable of delivering 7.5 amps @ 240V. (1800Va / 240V = 7.5amps)
From each hot lead, leg, to the midpoint neutral of each winding is capable of delivering 7.5 amps @ 120V. (120V x 7.5 amps = 900Va. 900Va x 2 windings = 1800Va)
L1 to neut 7.5 amps @ 120V.... L2 to neut 7.5 amps @ 120V.
For your balanced 60/120V (60V 0 60V) wiring configuration you must use the above 240V calculation.
The most FLA the xfrm will be safely capable of delivering will be 7.5 amps. 7.5amps x 120V = 900Va
So if each secondary winding is capable of safely delivering 7.5 amps @ 120V (as designed by the manufacture/size of wire used for the winding) then it stands to reason if both windings are connected in parallel (single ended) The xfmr would be capable of delivering 15 amps. 15 amps x 120V = 1800Va.