Tor-over-Tor is less secure than Tor by itself, because the circuits are not aware of each other and it is possible to reuse your first connections entry as you

Tor-over-Tor is less secure than Tor by itself, because the circuits are not aware of each other and it is possible to reuse your first connections entry as your second connections exit. i.e. A->-B->C, then the second connection becomes D->-E->A. This would allow A to de-anonymize you.