I train quad n-back so it would be hard to translate my training plan over to dual n-back. I suppose OP could do something like
10 trials per session 6 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
20 trials per session 6 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
30 trials per session 6 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
then:
10 trials per session 7 back
4 seconds per trial
(Move on to the next part if you are to achieve 80%+)
20 trials per session 7 back
4 seconds per trial
(Move on to the next part if you are to achieve 80%+)
30 trials per session7 back
4 seconds per trial
(Move on to the next part if you are to achieve 80%+)
then:
10 trials per session 7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
20 trials per session 7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
30 trials per session7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
OR:
10 trials per session 6 back
2 seconds per trial
(Move on to the next part if you are to achieve 80%+)
20 trials per session 6 back
2 seconds per trial
(Move on to the next part if you are to achieve 80%+)
30 trials per session 6 back
2 seconds per trial
(Move on to the next part if you are to achieve 80%+)
then:
10 trials per session 7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
20 trials per session 7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
30 trials per session7 back
3 seconds per trial
(Move on to the next part if you are to achieve 80%+)
I recommend that people replace the 6 and 7 n-back parts with the level they are on and are trying to progress too.