I am absolutely with Matt on this. Never, ever, pin the two aprons together! If you should catch the pin on a piece of furniture, or some other immovable object (and it is easily done), you will rip or pull the fabric of one or both aprons. Personally, I have never experienced this but, in my 55+ years of kilt wearing, I have seen several instances of badly torn aprons where the pins had been caught.

Few kilt pins are heavy enough to hold down the apron in a wind and they really should not be thought of as being anything other than decorative.

As for the position of the pin (the original question!): there are no rules and, again, I agree with Matt - let your eye decide where it should be. Usually, I aim for a spot about three inches in from the apron's fringe and about four inches up from the selvedge, but it really depends upon the tartan as the pin looks best when 'centred' between stripes.