No; that’s not an available feature (unless it’s been added recently). I don’t think it’s high on the list of features to add, either (though it’s not really that difficult).
I personally think that a secondary / backup MX is not advisable in many (or even most) cases. Keep in mind that virtually all mail servers will store the mail and re-attempt delivery later… any exceptions would most likely be from bulk mail senders, and even then, this is pretty rare. So unless you’re planning for your DSL line to be down for > 3-4 days, I wouldn’t worry too much. That’s not to say that I think they’re always a Bad Thing - just that you need to be careful and know what you’re doing.
With a backup MX, you often cannot enforce the same policies or reject mail to unknown users (unless you have direct control of it, or have some way of giving it a list of known addresses or users). This can result in more backscatter, plus more spam for you (especially since spammers, knowing this, often target the least preferred MX). On the flip side, if the organization handling the mail server for you has a more restrictive policy, you might have wanted mail rejected. If you misconfigure it somehow, you can actually decrease reliability.