you don't use a diode for low voltage batteries, scroll down on the link to see an example (not sure if its a viable circuit). I detected the existence of such circuits but never determined what they are/look like.
but I never saw it for a 2x 1.5V circuit, only for like multiple strings of series alkaline batteries that have a higher voltage (2x4.5). That circuit in the picture needs some higher rails I guess, since there is a comparator.
2x1.5V in parallel is kind of weird/unique, because normally... you just go up in cell size you know? 1xAAA -> 1x AA -> C -> D -> F. Then when you get parallel D or F cells for low voltage it gets funky I guess, but thats kinda strange ! A C cell can have 8500(like 7500 probobly)mAh, wheras 2AA = 5000?
IMO its kinda dodgy to see 2x aa in parallel unless there is a very good reason. And you must really not like power converters not to go 3V.....