ah seats...one of the few fields i have some expertise in
basically anything thats less than 590mm at the shoulders will fit side by side (you can go staggered if you want bigger seats but it means they will never fit right next to eachother...which isnt that bad actually since in practice the passenger seat is almost always set further back than the driver's seat)...now...in terms of your options they are many and varied and depend greatly on the size of your butt...
i would say if you are around a 32" waist you will have no problems with any of the Cobra line which are available from a mob called "revolution racegear" (they have outlets in MOST capital cities in australia)...they are all FIA approved but NOT necessarily ADR approved...which could be a bummer...the other option which i have actually decided to go with myself are the Sparco Sprints...they are 560mm at the shoulders and fit quite nicely in a mini, also are a little more comfortable for us with slightly larger waistlines (i'm a 34")...not the flashiest seats as its their bottom of the range...but they are sworn by by most of the motorsport pplz i know...seen some pics of some HORRIBLE crashes involving cars fitted with the sprints and in all cases the seats kept the driver safe...also...as an added bonus they are only $450 each

its also FIA and ADR approved...
here are some useful links:
http://www.cobraseats.com/ (i particularly like the imola2's and suzuka's...both will fit in a mini side-by-side but tend to be a bit tight around the waist unless you get the GT version which just happens to be too wide to fit in a mini

another drawback is that they start at $775, there is also the monaco seat which is fairly similar to the sparco sprint but with less room for your butt and i dont think it is ADR approved...i could be wrong though)
http://www.sparcousa.com/ (sprints baby YEAH!)