Gestalt

Gestalt is an option based on the 3.5 variant rule of the same name. It has some key differences, however.

Building a Gestalt Character
To make a 1st-level gestalt character, choose any two classes. Build your character according to the following rules:

Hit Dice
Use the average hit die of the two classes, rounding up to the nearest 2. For example, a gestalt wizard//barbarian uses a d(6+12)/2 = d9 hit die, rounded up to d10.

Base Attack Bonus
If the two classes have the low and high base attack bonus progression, use the medium base attack bonus progression for the gestalt character. Else, use the higher of the two progressions.

Base Saving Throw Bonuses
For each of fortitude, reflex and will, if one class has the low progression and the other has the high progression, use the medium progression. This progression is 1, plus 2/5 per level (rounded down). That is, it starts at 1 and increases at levels 3, 5, 8, 10, 13, 15, 18 and 20. Else, use the higher of the two progressions.

Class Skills
Take the average of the two numbers of class skills, rounding up if necessary. For example, a rogue//wizard has (2+10)/2 = 5 skill points, plus skill points equal to their intelligence modifier. A gestalt character with a class with 3 skill points and a class with 6 has 5. You treat all your class skills from either class as being class skills.

Class Features
For each class feature, you gain the better version from each class. For example, you cannot get sneak attack at a faster rate by playing a rogue 6//fighter 1/ninja 5. Usually, though, this means that you get the features from both classes. You do get the spellcasting progressions from both classes, albeit separately.

Advancement
At each further level, you continue to follow the same rules above when choosing classes. You cannot select a prestige class which advances multiple classes' spellcasting progressions or class features, or is otherwise intended to require multiclassing. Examples include mystic theurge, arcane trickster, and eldritch knight. The exception is if you can qualify for these classes entirely on one "Side" of the gestalt. For example, Cleric 3/Wizard 3/Mystic theurge 7//Fighter 13 is acceptable. Cleric 3/Mystic theurge 10//Wizard 3/Fighter 10 is not. Similarly, while you can take two prestige classes which advance the same spellcasting progression, they only do so at the faster rate, not at the combined rate.

Experience Adjustment
A gestalt character requires double the normal experience to reach a given level. The character still doesn't progress beyond 3,600,000 XP, and therefore does not progress beyond level 18.