If you connect 2 12v batteries in parallel that are a different state of charge yes they will become the same voltage but once you charge them in parallel then the batteries will be the same voltage/AH and the same if you draw a load of them, Now if you have got differnet loads of each battery then there is no point in paralleling them this defeats the object 4 battery terminal's become 2 terminals but double the A/H. In all the years I've been doing batteries I've never seen diodes connected to them. Something I've just thought about you've got to diodes in line yes so it will stop the on battery slightly draining the other(untill charged) but how would you charge them has the diode would not pass and you can't charge them has single units like I said otherwise there would be not point in paralleling