Except how many people know how to split the private key in the first place? Or know that there is actually no splitting involved but you're just taking two random PKs and combining them together?
Most of the procedures people use to generate a split vanity address involves such voodoo as generating a random PK somewhere and then combining them on bitaddress.org.
There is
one post that does explain what Bitaddress is doing, and I wrote it, but it has yet to be featured in anyone's software.
Not quite.
What I'm talking about is taking the other person's
part private key and generating a vanity wallet address - the result is imported by the sender (the result) into their own wallet. No-one else can import the found result into their own wallet. AFAIA no-one has found a vulnerability with this method and it was, or still is, being used and can be found in the various vanity wallet generating threads around the Forum.