Not without seeing them... Can you send me a picture?
My e-mail: MakeMineA10mm@hotmail.com
I guess at first blush, what I'd say is that it is probably a "put-together" set, and therefore a hoax. Someone could have had the G-21 engraved with "2 of 2" and had the slide refinished, possibly even by Glock to make it look good...
The idea behind the Defense Sets was that the owner would have matching compact and subcompact in the same caliber. Not only did this make ammo interchangeable, but it allows one to use the compact's larger-capacity mags in the sub-compact. (If you carry both guns, you'd just carry the compact's spare mags and be able to reload either/both pistols.)
One should never say "never" but I doubt Glock made a Defense Set in the same serial number range with a full-size 45 and a mid-size 40... Just doesn't make sense.
The G23 should be marked "1 of 2" and the G27 should be marked "2 of 2". The G23 should have a serial number between AAC0000 and AAC1499, so if it's lower, but has the same prefix that means it's a standard 40-caliber defense set item. Nothing unusual about that (other than it's a defense set gun...).
What is the G21 marked, as far as "1 of 2" or "2 of 2"? Also, does the serial number on the G21 match the serial number on the G23??