Double Sider 128 by Eric Mason is a multi-function utility designed specifically for the Commodore 128 (40 Column mode) and the 1571 floppy drive.  With this handy utility, you can take advantage of the second side of a 1541 formatted floppy without having to flip the disk.  There is built-in help, and if you know what you're doing, you might be able to use portions of this BASIC program in your own.