I would suggest for welding carbon steel to duplex 2304, an
ER 309L to assure adequate mechanical properties of the weld joint. You could use a nickel base filler but it must not contain niobium (columbium). For joining to duplex 2205 or the 25% Cr duplex to carbon steel use ER 309MoL.
For duplex to duplex - use ER 2209
No preheat for temperature at or above 50 deg F and keep the interpass temperature below 300 deg F. Watch your carbon steel thickness, you may need to butter the carbon steel with ER 309 if the code or standard requires a PWHT. Do NOT PWHT the completed weld joint!
This information was obtained from the following source;
"Practical Guidelines for Fabrication of Duplex Stainless Steels" by IMOA.