As far as I've heard from luthiers, there is little or no difference in durability between one piece and two piece backs. I haven't heard of them separating except in the process of making the instrument. Many violins by the old Italian masters (Amati, Stradivari, Guarneri) have two piece backs.
There's a good discussion here -- two of the people commenting in it, David Burgess and Luis Claudio Manfio, are world-famous luthiers with years-long waiting lists, so I assume they know what they're talking about.
Interestingly, you see more one-piece backs at both high and low ends of the price spectrum than in the middle. At the high end, one-piece backs are more expensive because they require a larger piece of high-quality wood without defects. At the low end, they aren't as selective about wood quality, so one-piece backs are standard because they require much less effort.
Funny how, when you think about it, people say, oh what a beautiful back, when you never really see it...at least when playing...when I bought my first real one they guy said it has a gorgeous back - just looked like shiny wood to me...took me about 6 months to discover they made 1 or 2 piece...
I did wonder, and presume that if violins were valued dependent on whether it was a 1 or 2 piece they may have been labeled as such way back, with one or the other being perceived as "better" or "higher quality" and those values would remain through today?
...and now I'm saying how gorgeous a violin is by looking at the back too...