IMO you should go 50 slash, 50 dw, 21 cs, 35 stealth and 24 envenom.
50 slash will help get the most out of your weaponskill.
50 dw will maximize the chance to swing with the offhand weapon.
21 cs will give you PA, although dual shadows (lvl 50 dw style) is just as viable.
35 stealth is the norm for scouts and infiltrators. 35+15 (from items and RR5) will give you an effective 50 stealth and mythic has stated over and over that going over 50 stealth will do nothing for you.
24 envenom because that's what you have left over, and although poisons will aid you in your fights, they won't normally win the fight for you. If you really want to bring your envenom up, drop cs down to 18 (you at least want garrote for the hinder effect) and bring envenom up to 26.