You have to be in co-op mode for this to work.

Get 2 ramp trucks and a Agency buggy. Have one player get in the buggy and drive on one of the ramp trucks and stay on the ramp end.

That means both wheels on the ramp and stopped at the tilted end of the ramp.

Then the other player gets on the other ramp and backs it to the other ramp truck without turning. Both ramps truck must be across from each other and their ramps must be facing each ramp so that means the tilted side faces the other ramp trucks ramp.

If done correctly all the vehicles will be flung and flying high.

Note: stay in your car and you should get the driving high achievement.

The buggy will fly the highest. The player in the buggy should start doing flips right after they get flung straight and you should do about 12 to 15 flips.

2nd Note: you might get the flipping crazy achievement so don't forget that too.

3rd Note: Make sure that you land on your wheels. If done correctly you will get so much driving XP that you would probably get your driving skill up.