If they are not self powered (no power cord) If you connected them directly to the speaker B terminals you would have no way to adjust them so the chance of getting them to work with your bookshelves correctly is pretty low. You will need an amplifier to drive them. The amp would be connected to the sub out of the receiver. You can use a stereo amp or a single channel amp if it is stable and powerful enough to drive both woofers in parallel.
If they are self powered you would connect them to the sub outs of the receiver directly. If you don't have two subwoofer outputs you can split it to connect both.