Connecting Rappel Ropes

I'm going to pick up a 7mm "tag line" to use for rappels with my 10mm lead line. I had heard that b/c of the difference in diameters the best knot to connect them would be the figure eight with double fishermans backups on either end.

Seems to me, though, that a flat overhand with plenty of tail on both ropes, combined with a double fishermans on the 7mm around the 10 as a back up would work fine too. Any thoughts?

There have been lenghty discussions on this site recently about how to join rope ends. Try this link

Thanks