Now, just so you guys know, this doesn't always work (only got it to work twice so far (I gave Trevors ATV to 2 randoms))
The people who I secretly told this too have confirmed it as working so just keep trying and you will get it eventually.

Glitch Requires 3 people

This is for giving it to your other character.

Requirements: have your second character's location set to property.

Step 1. Host an invite only session and invite your 2 friends.

Step 2. Go inside your garage and invite your 2 friends to it.

Step 3. After your friends are in your garage, You swap characters to your second character

Step 3.5(optional) one of your friends also swaps characters

Step 4. Get your friend (whose still inside your garage) to invite you back to the game.

Step 5. You should spawn in your apartment, quickly run to your garage and you should see the cars from your first character. Quickly get in/on one and Drive out before more cars fall and hit the vehicles.


This is for giving it to another person:

Step 1. Person whose giving away the vehicles (aka duping) starts up an invite only session.

Step 2. Invite your 2 friends to your game, then your garage.

Step 3. Host swaps characters to his second character, person who Is getting the vehicles also swaps character's to their second character.

Step 4. Player 3 invites player 2 back to the game.

Step 5. Once player 2 joins the game, he must quickly get into his garage, get in the vehicle of his choice and drive out before your vehicles spawn and touch the other vehicles.


For anyone asking, This is how you give Trevor's ATV to other people.

credit to VANAGON for the information

Towtruck
Space docker
Any Police car
bulldozer
lawnmower
utility truck
Any armoured truck/Stockade
Any prison bus
Jets/helicopters/tanks Cant go in garage
transit city bus
ANY VAN besides the youga van
box truck
Any limo
any army car/truck
trevors ATV
Dump Truck
Garbage Truck
Speedo van (thanks KILLSH0T707)
Mghill olsen truck (Thanks KILLSH0T707)
Any snowy car(Thanks Pubs)
Modded jb700
Vinewood tour bus(thanks tiptoegamer)
Tractor(Thanks tiptoegamer)
rusty tractor(Thanks tiptoegamer)
Cherry picker(Thanks tiptoegamer)
Bedless Rat loader(Thanks tiptoegamer

Any car you can drive into Los santos Customs
ATVs
Any kind of bicycle