I think we can consider running a controlled experiment to raise the limit to 2 simultaneously.
To bring some data to the discussion, I created a Data Explorer query to show the number of bounties set over time by month. I dumped the results into Excel, hit the graph button, and this is what I got:
Clearly, the number of bounties being set is generally increasing over time, but that is mostly due to increasing site activity (more people = more people using the bounty system). The January 2009 data is because the bounty system was only introduced to the site at that point; I'm not sure what happened in February/March 2010. It seems there's an upward trend after the new system was released, possibly with an "initial excitement" phase which will then level off a bit.
I don't think the system is escaping with nonlinear growth, so we can definitely consider increasing the maximum number of bounties to 2, even experimentally (say, 2-3 months). It would be interesting to see how many people actually take advantage of the feature.
At the same time, we have to realize that the number of bounties is increasing, which makes setting a bounty less "special" and attention-grabbing for those questions. That's why I think a controlled experiment may be in order. Or, perhaps Stack Overflow, with its "big-city" problems as Jeff calls them, may very well be the exception for increasing the limit -- it would certainly be safe to raise the limit on the other sites. With Stack Overflow, The Management may want to be more careful going forward.
EDIT: Added the total number of registered users over time. Here is the source query.
EDIT: Added the number of bounties set per 100,000 users. Sorry for the weird units, I wanted to get all the curves on the same graph. It's the trend that's important anyway. Also, I corrected the red line -- it should have been before the June 2010 data point since that point is the total over June. What I said above still holds, though, as the June 2010 data is about 2/3 old system, 1/3 new system.
EDIT: Updated with August 2010 data. It appears there is an upward trend in bounties being set, and it's slightly outpacing the rate of new users. We may need more than 1 additional data point to draw any kind of conclusion.
EDIT: Updated with September 2010 data. Rewrote the body of the post to give more current analysis.
EDIT: Updated with October 2010 data, including the trendline I added (but didn't post) last month. Revised the data analysis.
EDIT: Updated with Nov 2010, Dec 2010 and Jan 2011 data (man, I got lazy!). Astute users will notice the X-axis scale was corrected. The data points are now positioned such that they represent the previous month of data (i.e., the April 1, 2010 data point represents data between March 1-31). I think it makes more sense now.