merchants in Dubai typically will accept Qatari Rials, but unfortunately, its not the same in Doha.
For cabs and other incidentals, I think 200-500 AED/QR per day should be good.
Atms are plentiful in both locations, and most vendors do accept credit cards (but not the regular cabbies or small shops). If you want to exchange USD, check the rates at multiple counters, there will be a considerable difference. Even some of the hotels exchange at outrageous rates, considering both currencies are pegged to the dollar and don't change.