I have both XM and Sirius, and most of the channels and playlists are the same on both. i.e., the 80's personalities and songs are the same.
They haven't completely merged as far as the accounting goes, so you'll still have to pay the base of $12.95 (or whatever) on each. Second account is $6.99 on the base subscription.
I have two XM receivers. The first XM receiver is $12.95 and the second is $6.99. I pay a $12.95 base fee for my Sirius receiver. No word on when I can merge my accounts and pay $12.95 for the first one and $6.99 for each of the other two.