Why wouldn't you run the same team on both? Building two separate teams is not a good idea if you can avoid it.

Anyway - you need someone with Leech, and none of the champs you listed bring that. That's priority #1. Then you need someone to handle debuffs - stun of course is one of them, but unless you plan on only doing void CB, you also need to handle speed down. You probably should just go with a 4:3 built around Sepulcher, no need for Toragi. Probably Sepulcher + Skullcrusher + Rathalos + Draco + [Someone with Leech].

Was building two teams for speed tunes.  I was thinking, for full unkillable UNM team:

Maneater (unkillable/block debuffs) 4:3

Duedan the Runic (buff extension) 4:3

Pain Keeper (decrease cooldown) 4:3

Dracomorph (dec def, weaken, poisons) 1:1

Gnut (CA, Max HP dmg) 1:1

And I was thinking about a double ally protect/ca (1:1) in Life Steal for NM:

Bulwark (HP Burn, weaken, debuff extension)

Rathalos (dps)

Sepulcher Sentinel (inc def, blk debuffs, dec atk, def aura)

Toragi (ally protect, dec atk, poison passive)

Skullcrusher (CA, ally protect)

Here are my leech options:

Deacon Armstrong (fully booked and built)

Sachi (not booked but built)

Kunoichi (not built)

Vildrax (not built)

Itinerant (not built)

Temptress (not built)

why go to all the trouble of building two teams? just run a budget maneater comp (linked below) and build a second stun target to swap in for nightmare.

https://deadwoodjedi.com/speed-tunes/budget-maneater-unkillable/

If you insist on running two teams, Deacon works fine as a Leech person, just turn off his A3.

But especially given you have Deacon, you should strongly be looking at all of the various unkillable builds, as he makes things much easier.

I was thinking, with Duedan, unkillable would be up the whole fight so heals wouldn't be necessary at all...

 Also, my NM team is already built in Life Steal. Easy 1 key. Currently, it's Draco, Gnut, Sepulcher (using her aura), Toragi, and Skullcrusher. Really focusing on UNM Unkillable team and deciding who to bring in with ME/PK. 

duedan won't help you with that. unkillable buffs, as well as block damage and revive on death buffs, can't be extended.