There's no need to check in to both Providencia and San Andres if you don't want to. Just visiting one island is fine. I'm not sure why you've been advised otherwise.
If you want to visit just one of our islands, you'll find they are both very different. San Andres is quite package touristy and you will find plenty of supplies there. Almost all boaters who pass through here prefer Providencia though. It's quite a backwater but that is half of the attraction. An unspoilt, idyllic, undeveloped, paradise like Caribbean
island. Come and visit - most boaters end up staying here a while.