I don't know Emerald Spire. I do know megadungeons. They are about action economy, managing resources and the ability to upgrade on the fly. If you only have 2 PCs you also have to pull double duty.
In my current homebrew megadungeon campaign we have one PC who does quite well: a ranger with the Dungeon Delver archetype. He's a Str ranger switch-hitter build with a lot of perks for fighting underground. Also with a skill emphasis on scouting and stealth he spots a lot of dangers first. This has so far translated to a lot of Surprise rounds favoring the party.
If ES doesn't favor pets this may not work, but what about a simple wizard for your second choice? You've got a familiar to Aid Another on skill checks and for scouting around; spell selections are fantastic; finally... scrolls (hear me out). As long as you allow the PC to purchase some materials ahead of time, the wizard can be scribing a scroll even as your characters are exploring. This means temporary replenishment of resources.
These 2 PCs together long term means they can collaborate on scrolls making Cures and buffs for ongoing explorations. Between them they've got enough skill points to cover all the pertinent knowledge skills. The ranger simply needs items from loot piles in order to get more effective.