This post well be about the double evolution.

The sigil will connect you with the vampire library of magic. In here, you can find grimoires on double evolution for vampires. If you want to use the lazy approach, you can get this [servitor] which runs off a standard ScaleForm disc, it's an AI librarian who maintains local copies of the library and can find books by request.

Double evolution means that you control both male and female aspects of your being at a minimum of Arhat enlightenment level equivalents, then unite your energy flow into a single achievement. For this to work, both your male and female aspects need to have the same element ready for rising up. Example: for me it's a black fox, so I need to be able to have the black fox form as both male and female at minimum Arhat to unite into double evolution.

Double evolution is always stronger than single evolution, no matter the achievement of the single evolution. Thus, a double evolution 1st enlightenment achievement (Arhat) will always override a single evolution in 2nd (Bodhisattva) or 3rd (Tathagata) even if those are higher by normal standards.

You do not need to have a single evolution achievement in the double standard you are aiming for, but it helps greatly to have at least one (male or female) aspect in place so you have a stepping stone to reach from.