1.- Sponge and biomax will give you mechanical plus biological filtration, two sponges will also give you biological and extra mechanical filtration, at the end of the day sponges also make the function of a biofilter because they also can host a lot of denitrifying bacteria. Now, in what measure would two sponges provide more bio filtration than just one sponge and the biomax?....Hard to measure, we only know for sure that two sponges will help you to polish the water more.
I think in any scenario you will be Ok, so why not to try the two combos?. Run your AC with two sponges per two or three months and then swich to one sponge and biomax per other two or three months, just observe what seems to be the best and take your conclussions.
How often do we clean the sponge?.... It depends on your bioload and also in how often do you feed your fish, however talking about my own experience I clean the sponge once per month.
Cheers.