We have been double serving for the past year, with Google's permission. It's not technically double serving, as we only sell brand X widgets on one site, and brand Y widgets on the other site, so Google categorized it as different branding, and not double serving. The reality is that people are either going to buy brand x or brand y, so it was a good test.
After doing this for a year, we found that yes we got more customers, and more sales, however our advertising costs have gone up to mostly offset any advantage.
This year we are going back to a single adwords account, and are combining brand x and y on the same website.
The little additional profit we received from Double Serving was not worth the extra administrative overhead.
The person that is Triple or Quad serving, must have a high profit margin, and a low bounce rate, as paying twice as much across two accounts, gives you less than twice as many customers, with decreasing performance as you scale up.