Marion, I've been working largely on my Scottish line and most of them, but not all, have a single forename. What I have found is, those with a second forename usually are given it to memorialise someone. The parents are absolutely the same, so it's not a case of mixing them up with someone else. One recent .. oh, wait, I haven't actually added him yet .. still working down the sibling list .. case is my 3Xgreat-grandfather's nephew. When gggGrand's brother was killed by an exploding boiler, the sister back in Scotland named her next son after him. Brother's name was Robert, surname (and maiden name of the babe's mother) was Gordon .. so the boy was named Robert Gordon (last name). The only other member of the family (of that generation in that line) to be double-barrelled was the older sister of Robert Gordon .. Isabella Kerr. I have yet to discover where the "Kerr" came from (and whoever transcribed the original records wrote it up as Ken on her death registration).
So not every case is indicative of different parents.
The other thing I have noted is the tendency to have more than one forename after they emigrated to Australia. I cannot help but wonder if there's not some cultural thing happening there, or if it's more of the "let us remember those who went before". (Naming a daughter (girl name) (Wallace) (last name) pretty much says "I'm naming my child after her grandfather (or great-grandfather).)