How do you burn a single file on 2 disks?

You can use WinRaR or any other tool that allows you to split a compressed file into 2 or more files according to the size. And set the size as you can see it written on the disk
or if you are using power ISO go to Compress-> Advanced Properties-> then click Split to Multiple Volumes. The Volume Size will be now be open and put in how much you want on each disk. I.E 1000000K (1 gig) or 1,000M 1G then go back into compress pick the files you want to compress then they will be split into however many parts depending on how much you put in the Volume Size