Couple of thoughts Totally FWIW...
Tomos, you've mentioned that you don't like the button placement of the left-handed mouse... I've found over the years that choosing symmetrical mice (mouses?) make it much quicker/easier to move your hand from keyboard to mouse. And, a symmetrical design means you can easily assign buttons for right or left hand use with no need for a leftie mouse. I do realize that you're talking about a vert. mouse, but my point is maybe you should focus more on getting hardware that fits you rather than fitting yourself to the hardware?
As far as different button assignments go, have you thought about assigning sets per application? Logitech software for instance can detect the program and recall your custom settings. Don't know if that would work at all for you or not, and is based on the assumption that you wouldn't use both mice in the same program at the same time.
That said, both, current MS Intellipoint & Logitech Setpoint *I think* recognize different devices - maybe what you're after will work if the software you use recognizes both? Maybe they'd both have to be the same brand as the software so as not to show up as generic? Or maybe attaching one via PS2 & the other USB would do the trick, as long as the software had the info built-in to recognize the models? Other than very briefly while setting up a new system (where a PS2 mouse was initially needed at boot), the only time I use 2 devices is with my tablet, so I'm just guessing there - sorry.
Off-topic - Please Forgive - for those who are always wanting to be a little bit faster with their mouse...
I've found that the very thin, flexible, (I assume some sort of plastic material) kitchen cutting boards make really very ideal mouse surfaces! I've made more traditional looking pads by sandwiching a printed pic between very thin foam & one of these roll-up cutting boards, cut to size. Then I found a cutting board with a nice image, and a non-slip backing which I've been using for several months. And at $1 or 3, a huge discount if you've priced gaming alternatives. The downside is you have to keep it clean - any grit is immediately apparent, unlike cloth pads. The speed increase & overall feel is well worth it.
Some may also like this - I covered mine in foam backed material (for arm sweating), but even uncovered it's wonderful.http://www.dealextre...details.dx/sku.10541
Note: do not go by the pictures - the one showing the package has the correct use, as an arm support.