heeelllooo bruciebonus
if its internal and not required for external security{onto conservitory for example]
and the opening is square youve pretty much got the right clearence ok a mm or two to many but no probs providing you have a door stop fitted
if its for security plant 16 mm wide i think it is [dependant on your door furniture]
2mm less than half the door thickness on the internal door[the one that opens in first or out last] then form a rebate to fit on the other door
the 2mm less is to allow for draught proofing
if your realy clever and dont mind a 2mm extra gap to even out you can with a thin kerf circular saw [2.4mm or less] you can with just 2 cuts rebate the door and use the cutout piece to plant on the other door but its a skilled job cos you musnt cut to deep or youll have an unwanted channel or two on the door

the other option is to plant a bit 30mm wide by 12mm on the outside on the face off the door less secure and not so good looking
you could plant it on the inside but it wont give you the weather protection
if the doors have to be "fitted "[planed] to the frame and its more than a few mm youll be better adding to both doors as the the doors will start to look inballanced if you just plant onto one door
clear as mud