It's a monopoly if you want to listen to the radio is via satellites.
Of course as long as there is no laws are passed which prohibits it, any party could start their own satellite radio business. So the significant of the monopoly shouldn't be problematic, and I don't think it's in the public interest to hinder such a merger. Though I'm not sure if these two companies are going benefit in the long term- in short term one can be more efficient, because can reduce the number people it takes to run one company compared to two.

I think if you compare either of these companies [or both] going bankrupt, a merger looks like it could be the better option.