There's no gain in leveling the same class twice on the same account.

I'd recommand to start with 4 sham + protadin if you like both PVP and PVE. Since 4 shaman will be sufficient to own BG. And the team will also do well in PVE.

Once you get the hang of it, it's down to what you like. Prefer melee? prefer caster? Like a class specificly? Almost everything is possible.