I would recommend going for 195* internal temp, and then adjust from there to get your desired tenderness. Also, if you wrap the brisket in foil at about 160* internal temp, it will expedite the cooking process, and help keep the meat from drying out.
You don't have to separate the two parts, but it makes foiling easier if you do.
For a 15 lb. brisket, it usually takes about 12 hours for me, and I'm cooking at about 250*. The only way you'll be able to figure this out is with practice.
Swinestone Cowboys BBQ Team
FEC-100, REC TEC Grill, WSM 22.5", UDS, KCBS CBJ #54829