If you add both repos to sorces.lst that will cause major package conflicts unfortunately
e.g. gnomevfs package from Mer would override gnomevfs packages from Ubuntu witch would make it allmost impossible to install a nother desktops other than hildon that depend on gnomevfs. it would also likely cause other unstability problems within the system since some applications depend on package versions that are spacific to the os. Also, dispite Ubuntu being based on Debian, ubuntu uses a slightly different filestructure than debian. e.g. gdm for debian won't work in ubuntu due to some spacific differences in LSB.

Most of the deblet packages that i included don't have the following issues and that's why i'm including them in the install.