TMPGEnc DVD Author 1.6 (TDA) has an "Add DVD video"
button. You can keep using it to add to a DVD project until
you fill up the disk. The best way to do it is click on the blue
"Add new track..." text between your mini-DVDs so that they
will be separate titles/tracks in your new combined DVD.
http://tmpgenc.pegasys-inc.com/en/download/tda16.html
for the free demo.
Luck;
Ken