If use two discount coupon codes with the same discount type and make the "Cumulative with other discounts" checked, then only one of them could be applied.
We should allow to enter multiple coupon codes. The question is how we should store it? The current approach is not acceptable
Source: http://www.nopcommerce.com/boards/t/43403/discount-with-coupon-code-applied-not-corrected-and-usage-history-not-updated.aspx