It's an idea I've done, seen done well, seen done badly, and all parts of the spectrum in the half of my short life that I've been RPing for.
The idea works very well if there's some mutual trust between the players and good communication when it comes to building an organic backstory (with elements often being ironed out as they come up on-the-fly).
I'm far too occupied, but I like the idea and have enjoyed it in the past, I'd just suggest that you choose somebody you have good synergy with as a writer.
Edit: I'm guilty of only flicking through the thread and I see that Desi shares very similar sentiments to me.
I'll go one further then.
Both players need to iron out and be in agreement of the backstory as I pointed out. This means you should be prepared to relinquish some of that creative space, effectively meaning allowing someone else to effectively mold some of your character's past. This can be an issue for some people. Whether it is to you is another matter.
As I mentioned '... on the fly'. You'll be RPing and some question will come up - the answer to which you haven't fully tacked down - something which effects 'new character' and Silas. In this case you'll likely have to shoot a quick PM at each-other. To this end it helps to be available at similar times. This becomes especially collaborative with those 'eureka!' moments where inspiration strikes for some tidbit you can work into your character's back story seamlessly, as some further justification for how they've shaped as a character.
These are what you could consider the negatives - but they're still not strong negatives. It all comes down to synergy, availability and organization.