mix [bottle 1] and [bottle 2]
HM Assassins of level 45 and higher can take two poison bottles of any kind and mix them together. The mixed poison, when used, will perform the two actions of both the poisons. For example, if a blue and black poison are mixed, when injected the victim will suffer the effects of one blue dose and one black dose.
Poisons can only be mixed in HM labs and there is a delay between the manufacturing.
Due to the volatile nature of some of the chemicals used in them, (read: Balance Concerns) not all poisons can be combined.