@grannyring will surely weigh in, but in the meantime, getting two runs of 12 awgs opens up at least three possibilities:
1. simply replacing the bass run;
2. twisting together with the 16 awgs (effective awg 11) for the bass run;
3. twisting the two 16awgs together for the head run (effective awg 13), and using the 12 on the bass.