You could probably email Clarion or Kenwood and ask them what effect it would have on the tuner to splice together two antennas if you so desperately want an answer. I highly doubt anyone else has done anything similar. I most particularly doubt someone on this forum would know, so if you didn't want speculation then why post at all?

The satellites broadcast between 60 and 90 degree elevations, and about 85% of the time there are two Sirius satellites in the visible sky, so as long as you have the antenna in a place that views the sky from 60 to 120 degrees then you'll get signal. My front dash windshield provides that angle. Two antennas won't help you that often. What will help more is buying a tuner that has a larger buffer stream so if it does cut out for a couple seconds the tuner has the broadcast already in memory. My Panasonic tuner only has a 3 second buffer, which is fine for highway overpasses and all, but I'd prefer it to be larger.