A second Ring of Wealth can contribute no more than 2 levels (i.e. a +1 ring) when calculating wealth bonus for dropped equipment equips. This is to prevent it being possible to endlessly increase Ring of Wealth level by farming pairs of high level Rings of Wealth.
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.
Best strat is to get a +4 from the imp, reforge in another 2 rings of wealth and apply 15 scrolls of upgrade. Then have another +1 for a total of a +23 on your ring of wealth. With other rings you can get up to +24 if you have a +2 instead. You can boost this up with the rouge talent that adds +3 levels on your ring for a total of +26 levels on your ring of wealth, +29 on any other ring.