Don't claim to be an expert, and have never tried this, but in a perfect world, it should work. The problem is that this is not a perfect world, and how you wire them up is only a small part of that imperfection. The two 6s likely have a different capacity than the 12, and since even copper wire has some small resistance no matter how large that wire is, I can forsee lots of problems with keeping such a system operating well. But what do I know, other than that common wisdom suggests one ought to only hook up well matched batteries in parallel, and do so with careful attention to not overcharging one while undercharging the other, or discharging one excessively relative to the other. I think you need to do a lot of reading on the topic before you try it, not merely pop off a query to an internet forum, where you are guaranteed to get lots of advice on both sides of the issue from lots of people with no real knowledge of the topic. Like me.
