You'll have to split the input signal, if you are using RCA. Head over to Radioshack to pick up a mono RCA to dual RCA splitter.
Thanks. I'm not a Radioshack guy but I did go to my local electronics parts supplier and asked just what you suggested. I was going to use the spare tape-out (recording) jacks for my level in to the sub. However, by doing this you have to adjust the volume of the sub every time you change the volume of the receiver. I tried just using the wye as you said and the volume level of the sub will not change, it must be done independently of the receiver.
What they suggested, and it works, was to purchase a line-level converter ($20.00 + tax) to connect to the speaker lines out of the receiver and then wye out of the converter into the single port of the sub. That way the sub had an un-amplified level in and the volume of the sub would go up and down in unison with the volume control on the receiver. Although you do have to experiment a bit with the volume control on the sub to establish a level that suits you at a particular volume level to begin with.