I had the single line setup but the problem that drove me to the two line, even when Uni, was that after several sets and douses, either the halyard or the tack line would eventually get 'kinky' and foul a set or douse. That and I find it much easier (less drag) to quickly pull up the halyard after the tack has been pulled out first, as was said earlier, it pulls the spin half out of the bag for you.

On the douse, I've been in situations where I was glad the tack line was still out (crash gybe in the middle of snuffing) as it kept the spin from going under the bow.

Like Robi said though, time on the water is the best cure.