LotR 2nd Edition Boxset 2nd, 1st, 1st or make all 1st?

I suppose you are talking about a custom made box? The second edition books with grey covers have not been published as a boxset.

No, the US Second Editions were published in a boxset. Sorry, didn't specify US Edition.
Though looking at this, apparently there was a 1, 1, 1 boxset, thought every one I've seen has had FotR one later printing than the other two.

https://www.tolkienbooks.us/lotr/us/hc/hm1967/lotr-slipcase-1967