Crash Bandicoot: The Wrath of Cortex - Level 27: Ghost Town (Double Gem)

After getting 10 Relics, you unlock the second extra level. In Ghost Town, for some strange reason, you're racing against Crunch on mine carts in an Old Western town. This is the only non-boss level that you face him in. If you lose the race, you can start over without having to go back to the Warp Room, first.

The first gray Gem is won by beating Crunch in the race. Certain turns on the track can keep you ahead of him with some speed boosts, because just tailing him from behind won't do anything.

The second gray Gem is won by breaking all of the crates, of course, but this level sets them up in a way that you have to use all of the branching tracks in the level in order to get them all.

The most important thing about this is the first "!" Crate you find. Hit that in the first Lap to turn on a second "!" Crate. Hit that one in the second Lap to turn on a third "!" Crate. Hit THAT one in the 3rd and final Lap to turn on a Green "!" Crate. Then finally, on that same Lap, hit that Crate to blow up all of the Nitro crates in the level before you finish the race, and that second Gem is yours!

It is entirely possible to get both of the Gems in one sitting, here. Just watch this video and see for yourself.