iOS Dualboot

There is almost everything I know about iOS dualbooting. This text was written with hope that information given in it would be useful for someone. All operations in this guide are performed under OS X, but should be possible on other operating systems too. Also, don't forget that only iOS 6 and 4.3 (look into Addition 3) are considered as second system here
No warranties given, follow this guide on your own risk, you are on your own

List of contents

Part 0: Requirements

Part 1: Preparing RootFS image

Part 2: Partitioning

Part 3: Restoring RootFS

Part 4: Modifying filesystems

Part 5: Patching bootchain

Part 6: First boot

Addition 1: Jailbreak

Addition 2: Alternative untether method

Addition 3: iOS 4 dualboot

Addition 4: iOS 7+ dualboot

Addition 5: iOS 5 dualboot

Credits/Thanks to

@danzatt - for original partitioning guide and hfs_resize

Roderick W. Smith - for gptfdisk

@winocm, @xerub - for kloader

@iH8sn0w - for help with keybag issues

@JonathanSeals - for iBSS patches, multi_kloader fix and baseband firmware fix

@kdolganov - for help me with testing few tricks

@ShadowLee19 - for NOR-flashing untether method

@axi0mX - for iOS 4/5 compatible kloader and firmware keys for S5L8942 devices