[...paralleling two primary windings of a
transformer...]
Just make *very* sure you get the phasing right; swap the ends of one
of the primary windings and you have a close approximation to a dead
short across the incoming power feed. Something will blow; if you're
lucky it'll be your mains fuse/breaker rather than the transformer.
A useful trick is to connect a 100W mains light bulb in series with the
input to the transformer when first applying power (and with the
secondaries of the transformer unconnected). Such a bulb will easily
handle the magnetising current of a small transformer, anf won't even
glow (or drop much voltage), so you can check the output voltages, etc.
But if you've got something wired up backwards so you're effectively
putting a dead short across the mains, the bulb will limit the current to
something reasonable (and tell you of the problem by lighting up).
-tony