For issues like this, when I do it, I take the photo through a photo editting software, whichever you prefer and cut the image right down the middle. Then, like what Dillinquent posted, I will set a percentage for how big I want the image then make sure it will take up 2 pages. Also, I will make sure that the pages line up so that if I have my iPad in landscape view, then both images show up as 1 taking up both of the pages present.
Seems to work good for me that way. Might seem like a lot of work, but I only have to do it once, then once the ePub is saved and backed up, it never has to be done again
P.S Also, to make it look like one big map, I set the margins to 0 to make sure there is no white space in-between.