Or at least that island’s making it “easier” to tear them apart in stamp form. I’ve never seen a stamp designed like this. Tell be philatelists of the Dope, is there a name for this type of stamp or is this something Niue came up with on their own?
Do you mean the sort of design where two different stamps form a single picture?
How about the 1967 “Accomplishments in Space” commemorative at the top right ofthis page?
There a lots of other examples from all over the world. The only “unusual” thing about this issue is that it splits the couple in half - each to their own stamp. This makes superstitious folks go “hmmmmm”
Or, further down on Euphonious Polemic’s link, the 1981 “Space Achievements” issue, which has a single picture spread over eight stamps. I remember buying a few of those when they came out, and putting them aside as future collectibles. Haven’t seen them in years, though.
Huh??? Together they are worth $4.30 but if you separate them they are worth a total of $5.80?! Does that mean I can buy a $4.30 stamp and turn it into $5.80 worth of postage just by tearing it in half?
can I do that with a twenty?
Um, I suspect not.
Why would you want to turn a twenty into $5.80 worth of postage? 
You make it up in volume.
Well, they are the only country in the world that is producing an attractive commemorative stamp issue for the royal wedding.
What?
How many?
Never mind.
Lots of silly stuff make superstitious folk go “hmmm”, that’s why they’re superstitious!
I am a philatelist, and I always chuckle when I go to look for new stamps for my collection and I see how ubiquitous stamps are commemorating some royal marriage of the 1930s or 1940s (can’t remember which, and I don’t think I have them in my collection, despite my preferring the old stamps, since they are so ubiquitous and cheap) - I think every commonwealth realm at the time had exactly the same stamp issued in honour (only changing the name of the country).
As for the William/Katie Niue stamp, again, as a philatelist, I didn’t bat an eye, seen the device used before, and found it rather clever way to have a usable design should one with to separate them. I’m half tempted to get a pair myself, but no, it doesn’t make me want to travel there. 
(I have stamps from I’m guessing over 300 countries, past and present, it’d be impossible for me to travel to them all merely because I own a stamp or twelve from there - wishful thinking indeed, Niue Tourist Board!)
Probably has something to do with the alimony she’ll receive after the royal divorce.