Congratulations on making it this far. Now, your final test begins. I have a complete items list... just take everything on the list, and then use certain items in the order i give them to you.
This is the list:

Book of Combos 1,2,3
Potions 10
Mega Potions 10
Mega Nutrients 3
Mega Demon Drug 1
Gun Poweder 10
Well-Done Steaks 5
Paint Balls 99
Flash Bombs 5
Net 2
Trap Tools 2
PitFall Trap 1
Lg Barrels
Sm Barrel Bomb 10
Lg Barrel Bomb 3
Herb 3
Max Potions 2
WhetStones 20

Ok thats the list... i think that should completely fill your inventory.. so your wondering where is the map etc gonna go right? well this is what ya do. When you start the quest, Drink the Mega Demon Drug, its abbilities last for the entire quest, then drink 3 Mega Nutrients then eat 3 herbs. Take out the rations and eat those. That should leave you with enough space for the map, F.A med, and what ever else.

After all that is done... how you take on the quest is your choice, Rathian is in area 9, Rathalos in area 5. The only area they are both in at the same time is area 3.
What i would suggest is using all your flash bombs on Rathalos, and all your traps on Rathian. When it comes to the bombs.. i'd say share them evenly.

Good Luck!

as for the bullfango chasing you into area 4, that area cant be cleared anyway.. so try to get used to being able into area 3 from 5.

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What do we do when we wake?
Keep both eyes on the Sky.
What do we do when we sleep?
Keep one eye on the sky.
What do we do when we see them?
Dig hard, dig deep, run for shelter, and never look back.

Reign Of Fire