Yeah, like other people said if it's not a horrible amount of trouble to mount both, two is better -
1. With one on steady and the other on blinking you can get the best of both worlds - one blinking which catches attention and identifies you as a bike to drivers, the other on steady which makes you much easier to track where you are.
2. If one fails people can still see the other one. The problem with the tail light is once you start riding, if it stops working you don't notice - it's behind you, you can't see it. I've arrived home before to find that the batteries in the light died, or a battery got jiggled loose (though I either permanently fix it or get a different light then) and I never noticed during the ride.