To me, it would seem your easier option would be to split the content between
two letter-sized slides on the front end rather than splitting it on the back
end using the printer.
Some printer drivers will allow you to print over multiple sheets...but I
suspect with 99% of them the division will be a default and will not fall
where you want it to be (it'll be in the center of the slide).
I am curious though...as you'll have to trim the spine even in your
requested setup, would it make a significant difference to you to cut it out
of a legal sheet (with the cover on it)? This is certainly the cheaper
option
Cheers