Need to get a RCA splitter to split the LFE signal. Run one of the two ends to your original sub. Then you will have an extra one, run this to the second sub. You will need to run avia or some other set-up program to tune the second sub.
Depends on what you want. If you want the second sub on the .1 Channel as well than you can use a Y splitter like David suggested. Remember that in getting the best results you have to buy an identical one. You can also connect it to the front/surround channels. It's up to you.