With a hard or symbolic link, this may be possible, but why not sync it using a sync service, which may be more reliable and wouldn’t break if you needed a second device or yet another user account (which would require a new link).
With Dropbox, can’t you share the Alfred preferences folder out to another Dropbox user? (I admit I haven’t actually shared folders out of Dropbox so I don’t know how well/reliably this works.)
I imagine The preferences could basically be put anywhere, including in a local folder that multiple user accounts on the same computer can access, (say, in Macintosh HD/ with the correct permissions)