Unlike the Toyota/Subaru twins, these two cars will probably be quite different. Both the engines used and some of the body elements will separate them and give them individuality.
Alfa is likely going to use a de-tuned version of the 1,750cc turbo engine from the Giulietta QV, while Mazda will either use high-compression naturally aspirated engines or a hybrid system. Rumors about smaller turbo engines should pretty much be ignored, since Mazda officially announced it wants to forgo downsizing trends. One of the photos you'll find in the gallery below actually shows an RX-8, which could mean they are using a rotary engine.
Both cars are due in 2015 and will likely be produced on the same assembly line in Japan.