With a single sub, the sub will sum the signal on the red and yellow leads, low pass filter it, send it to the internal sub amp, then on to the speaker. With two subs I see two possibilities:
1) Attach the red lead from one sub to the right amp channel output positive and the red lead from the other to the left channel amp output positive. Black lead on each sub to its respective amp channel output negative. Yellow leads unused.
2) Attach the red and yellow leads from one sub to the right amp channel output positive and the red and yellow leads from the other to the left channel amp output positive. Black lead on each sub to its respective amp channel output negative.
Because the sub sums the signal on the red and yellow leads, option 2 will result in each sub being about 6db louder than option 1. It may result in too much bass, but experiment and see.
1) Attach the red lead from one sub to the right amp channel output positive and the red lead from the other to the left channel amp output positive. Black lead on each sub to its respective amp channel output negative. Yellow leads unused.
2) Attach the red and yellow leads from one sub to the right amp channel output positive and the red and yellow leads from the other to the left channel amp output positive. Black lead on each sub to its respective amp channel output negative.
Because the sub sums the signal on the red and yellow leads, option 2 will result in each sub being about 6db louder than option 1. It may result in too much bass, but experiment and see.