Your receiver has "pre outs" on the back. Get subwoofers with right and left line level inputs.
You can connect one subwoofer, via their line level inputs, to each of them. If you want, you can "split" each signal with a F/M/M "Y" connector to feed both R/L inputs on each sub, but it's not necessary.
Or, you can use one sub with right/left line level inputs.