It is very difficult to accurately calculate this probability. The main difficulty lies in the fact that many of the
hands from which these combinations could form will be folded on different streets. It is necessary to take into account all the options for choosing 2 players from all - this significantly increases the likelihood of seeing such a confrontation at the table. But you can calculate it very roughly by simply multiplying the frequency of the straight flushes by the frequency of the quads, taking them from the tables for Hold'em.
For 6-max:
P = p(s.f.)*p(qds)*C(2, 6)
C(2, 6)=(6/2)*(5/1)=15
For FR:
P = p(s.f.)*p(qds)*C(2, 9)
C(2, 9)=36
