Nostalrius Begins - Quality wow vanilla realm (1.12)

Originally I made a character by myself on this server and things were going as per usual.

But when I got my friend on here and we started leveling as a party we are just constantly falling behind on exp to the quests n now are 5 levels behind what we should be.

Is there a reason for this?
Is there a way of getting around this so I can continue playing with my friend?

pretty certain its because you're now splitting exp therefore only gaining half as much as you would when you're alone killing mobs. you'll just have to farm twice the amount.

additionally, you need to level in multiple zones of that level range and not just one otherwise you will "fall behind"

Hmm sounds weird, what lvl are you and what zones have you been doing? I've been lvling an alt to 18 on horde side always grouped with 2 mates. Us 3 haven't had this issue at all while in Mulgore and The Barrens. Lots of quests are based on drops from mobs, requiring us to kill 3x the amount of mobs for those quests.

if u didnt play vanilla until now and ur kinda noob for vanilla (im not saying u are but if its ur 1st time) you should be doing ALL the quests in your zone not skipping any quest and should probly google if there is any left and if its going that bad i suggest waiting a few days every now and than for making your rested xp maybe than u will get better exp

It is not, that you re leveling as duo. This is vanilla, you just gotta be questing in multiple zones as someone said before.
For example ..... you got red quests in redridge mountains ? Go ashenvale, you got red quests in ashenvale and redridge ? go stonetaon or wetlands... And when your quests in redrgidge are yellow get back there. Its just working like that.
I am usually doin 2-3 locations at time and dont have problems being with falling behind.