I asked around, and here was one response from an car audio specialist:
The future is the XM network, so basically the Sirius network is
'as-is' and only providing limited hardware. The SCC1 tuner is
discontinued and it is required for the Sirius kits.
It looks like the ISNI572 kit should work with your Altima, provided
it doesn't already have satellite radio, but also meets this criteria:
1> Radio must have a "SAT" button or have "CH", "CHANNEL", "CAT" or
"CATEGORY" printed somewhere on the face of the radio to be
compatible. This is usually located near the Tune or Seek buttons.
If you already have the SCC1 module for some reason, then you would
just need the iSimple NI2 kit with the satellite radio cable and
probably an antenna of some sort (or an adapter if it has a satellite
radio antenna on the roof already).
Vehicles that don't meet the criteria above, or have navigation, are
automatically excluded. Many vehicles have the XM receiver module
inside the in-dash radio now, instead of as a separate box, which
makes many late model vehicles not capable of conversion.
Also, it looks like SiriusXM will let you combine different radio subscriptions on one account now (see this link); even when one radio is Sirius and the other radio is XM. I haven't actually tried this yet, but I will soon when I can no longer tolerate FM radio (very soon).