I can't tell you the name of the company or the schedule, but I can tell you that there are at least 6 buses a day from Samana To Santiago direct...I found this among some old lists I have tucked away:
Linea Junior
P.F. Bon? #69
809 582 2134
Las Terrenas
6:45
11:30
3:00 p.m.
Saman?
7:15
8:15
9:30
10:30
1:15
2:30
3:45
4:45
Costo Julio 2009: RD$250
I guess you could call them about the reverse trip???
HB