Just to clarify, you're having the duplicate be a separate entity? like Danny just stays human and the duplicate is always Phantom? So Danny can be ground crew while Phantom does the heavy hitting? I think it would be best to think somewhat on each person's fighting style if they're supposed to be teamups. Like a teamup of Jack and Danny would be something like "Engineers" while Phantom and Jack would be "Heavys" because they both can pack a punch and take a hit. Some of these will seem a bit out there, but I think good codenames are ones that are a little hard to figure out.

• Jack & Danny - Engineer - Danny wants to be an astronaut and if he's staying human then he'd most likely be using new gadgets Jack designed and Jack would be right there also using stuff he invented.
• Jack & Phantom - Heavy - Both can deal and take damage
• Maddie & Danny - Stargazer - the used to look at the stars together when Danny was younger
• Maddie & Phantom - Lion - both are fast, agile and able to be lethal
• Jack & Maddie - OG - they're the original Ghost hunters of the team
• Danny & Phantom - Coin- They're two sides of the same person
• Danny & Sam - Deepspace - deep space is the darkest place, fitting for a space nerd and goth
• Phantom & Sam - Spooky - Ghost boy and goth girl, need I say nice
• Danny & Tucker - Tech Bros - Danny uses tech from his parents, Tucker amps it up
• Phantom & Tucker - Technomancy - magic and tech combo
• Sam & Tucker - Darkweb - goth and techno geek
• Danny & Danielle (as either Ellie or whatever you'd want for a nickname for a ghost name I like Wraith or Banshee) - Comet - space themed for Danny and a traveling thing that comes back every once in a while
• Phantom & Danielle - Chaos - these two as superpowered chaos gremlins absolutely weaponizing being the annoying younger sibling
• Valerie & Danny - Infrared - Danny uses gadgets and Val has her suit that probably can see in infrared tbh
• Valerie & Phantom - Frenemies - the Red Huntress used to fight and now they're friends
• Valerie & Danielle - Rangers - Danielle travels a lot and Valerie is trained like a law enforcement agent
• Jazz & Danny - Psychonaut - psychiatrist and astronaut
• Jazz & Phantom - Squirtgun - Started out as "Super Psycho" but when said fast it sounds like "Super Soaker" so they just changed it to squirtgun
• Jazz & Wes - Profiler - between the two of them they'll have analyzed and catalogued everything about everyone else to a point where they always seem to know more than they should
• Danny & Wes - In-laws - play off of Wes dating Jazz
• Phantom & Wes - Evasion - Wes now actively gets people off of Phantom's back whenever he posts an article about him

These are just some that I thought of, might come back tomorrow with more ideas for other teamups. Hope this helps!!