From all that I have read, to get the best percentage, is to upgrade one ring as high as possible (one person mentions +15 being the best possible for drops) and don't bother upgrading the other, the second ring gives a bonus, but no additional by upgrading it.
I have only had a couple games with two rings of wealth, so I haven't done much testing.
See what happens when you can't find the right bumper sticker with the message you want to share?