Changes

Due to difficulties with filming, and finding actors, I've decided to change my idea slightly to allow filming with only 1 person as a pose to 2. 
The pieces of writing in bold and underlined show the changes to the narrative plot, in order for me to be able to film with just one person.

The film starts with Brian laying on a field. 
The screens goes into black and white, stops at the image on screen and Brian's name pops up above his head. When his name pops up, the camera slowly zooms out to give the shot a little action. A couple of seconds later, the screens goes back to colour, and a high angle tracking shot in a full 360 circle reveals Brian (2) laying, and the screen stops, goes black and white, and his name pops up. Again as his name pops up, the camera slowly zooms out. 
The film cut's to a long shot of  both Brian's laying together, and one of them starts talking.
"Dude, Where is everyone?"
Another cut to a high angle, and then the other Brian replies.
"Everyone is gone..."

The way I will do the above is to have one angle showing Brian in full colour, and then as Brian (2) speaks, I'll cut to another angle in black and white.

A cut to medium shot as ambient music starts playing in background and the guys get up and wander across the field (camera panning) and out onto a nearby street.
As the guys disappear the camera cuts to a medium low shot from the street, to see their feet and legs appear.

In the scene above, I will either cut back and forth between black and white angles, and full colour angles to show the different characters.

Long shot pans across the roof of a car to show the guys walking up to it, and start talking.
"Can you drive?"
Camera cuts to close up behind them as Brian (2) replies.
"No... can you?"
Camera cuts to close up of Brian (1)'s face, as he replies.
"No... I guess we're walking then"
Musics start to play louder as they wander down the street. Camera cuts to extreme long shot in front of the guys, walking towards camera.
Brian (1) reaches into his bag, to find nothing inside (close of up inside bag) and asks:
"You got that drink?" only to find Brian (2) has disappeared.
The music stops. And camera tracks around Brian (1).

For the conversation scene, I can do a shot reverse shot of the one actor, showing the back of his head as one character, and then his face as the second character.
Another way I could do it is to record the same person saying both parts, and then split the screen to overlap the conversation. 

"Hey, where did you go?"
The sound of a heartbeat fades in, getting louder. Camera cuts to medium shot looking out of bushes towards Brian (1).
"This isn't funny!"
Brian (1) starts to walk, and gradually gain speed until he is running. Camera cut's to medium shot in front of Brian (1) and pans fast as he runs past. Just before he runs past camera cuts to his point of view, and the cuts back to match on action panning where there are both Brian (1) and Brian (2) running.

For the scene above, I can do it 1 of 2 ways.
1 way to do it, is to film the actor running twice in slightly different places and overlap the footage.
Another way is to have the screen turn black and white after the point of view shot. This won't be as visually pleasing, but it will be easier.

"Dude, why are we running?"
All heartbeat sounds stop and camera cuts to close up as both runners halt.
"Hey man, I need to tell you something.. but you might want to sit down.."
Camera pans to view a bench, as the guys walk over to it.
A loud thump sound plays as Brian (1) sits down. Over the shoulder shot from Brian (2) looking at Brian (1) as Brian (2) starts talking:
"Well I don't know how to tell you this... but you suffer from multiple personality disorder."
Over the shoulder shot continues throughout the conversation:
"What... What the hell does that mean?"
Brian 2: "It means I'm not real.. I'm just you, well... I'm me, but I'm you. It's all confusing really.."
Brian 1: "Wait... What?"
Brian 2: "I'm your other personality, I don't exist. But, I have more bad news..."
Brian 1: "Bad news? Judging from the absolutely jolly news I've just heard can it get worse?"
Brain 2: "Seeing as I'm not real, you are technically the last person on earth."
Camera cuts to medium shot as dramatic music starts to play, gradually getting louder.
Camera starts to pan round in a 360 circle and when the view gets back to Brian (1), Brian (2) has disappeared.

Again, For the scene above I could just do a shot reverse shot of the same person, switching between face shots and back of the head shots for each bit of the conversation, or have the screen split and film the same actor talking twice for each character's part.

Music stops, and heartbeat fades in. Fast moving shot following Brian (1) running up the stairs of a tall building, and then cuts to long shot on the roof of which Brain runs out of the door and towards the edge.
Heart beat gets louder and louder and fades to black as Brian jumps off.