Will this work? AC transformer, 15V 1ampere, center tap, voltage drop
With your chosen resistors you have a 15 Ω load on each 15 V supply. From Ohm's Law you can show that this will cause 1 A to flow through your resitive divider. Now you have 100% load on the transformer and nothing left to power anything else.
You could double the resistor values to reduce the current but when you connect a load in parallel with the 12 Ω resistors the voltage will fall due to the increased current in the parallel loads.
Your options are:

Buy the right transformer.
Remove 20% of the secondary windings.
Tell us what your load is and why you want 12 - 0 - 12 and we may be able to suggest alternative solutions.