Need help with Character backstory

[GreatCthulhu]

So I'm one of two people in my D&D5E group who can't make every session. Our DM wants us to come up with an idea to help explain why our characters aren't always there. But we don't want to be bound to the same race/class (I'm eyeing playtesting the pugilist and seeing if I can be a Minotaur). The two idea's that we've come up with is 1) that our two characters got combined somehow, and sometimes one takes over and the other disappears (kinda like Bruce Banner and the hulk). 2) we do Ladyhawke where one is an animal when the other is human or whatever.

Personally I like the first idea more. But I need help coming up with a story/reason how our two different characters became linked in such a way. Is there anything in Forgotten Realms lore where something like this has happened before? A spell/curse/wondrous item? Any help would be appreciated.

[Specter]

When I did it, I blamed it on family problems. My in-game father had been affected by the plague we were investigating. Worked for me.

[JellyPooga]

Tricksy one...

I would see if the GM is willing for the two of you to share one body, but play different Classes. Two minds in one body. A Curse, perhaps? Could be interesting to turn up to a session half-dead or altered in some way (Reincarnation could be especially amusing!).

[Hrugner]

Double life works out pretty well. Have your character be an emissary to another plane if the coming and going is likely to happen in the middle of adventures.
"So, the thing is, I'm a genie trapped in a lamp and your world is inside the lamp. Whenever some schmuck rubs the lamp I pop out and grant him a wish. I'm afraid I can't grant wishes inside the lamp though and you never know when someone's going to rub me the wrong way."

If it's likely to happen at the beginning and end of adventures, then being secret nobility, a very busy merchant, spy or criminal works out better. Less silly at any rate. You could also suffer from some sort of disease that leaves you comatose most of the time, but you'd need to be pretty unique to justify being hauled around in case you woke up. Or perhaps your character is very powerful, but tends to burn out quickly and needs to rest. A warlock would be fairly easy, you could just have your pact deal involve being taken away for some task. Time traveler always works. Animated statue that triggers randomly should be fine.

I think competing adventurer may be the easiest. You're sort of a lone wolf type who occasionally groups up with other parties, but always with disastrous consequences. The only group that doesn't seem to die and leave you hanging is the actual party who you reluctantly work with whenever they inevitably show up.

[lordarkness]

Sounds like one could be the host and the other a being that possesses that host.

That being could be an intelligent weapon or item, a ghost, a being from another planet, a curse, a parasitic beings, a spell gone awry, a voluntary choice to help save a friend whose body was just destroyed, etc., etc. etc.

Could be fun.

[Tallis]

Sharing the same body could get awkward if you're ever there on the same day. Maybe you're suffering a curse from an extraplanar being who is periodically able to pull you into their home plane to serve as their slave. You are somehow linked to one of the regular players so that when you escape you always appear near them. Maybe they wear a ring that was once yours? Make it an object not the person himself so if something happens to them someone else in the party can still carry the object. On a similar note maybe you are a figurine of wondrous power but with random activations. The party can't control when you're summoned to life but they carry your statuette around because you're awefully useful when you do appear.

[Gastronomie]

As already mentioned, that idea will eventually get problematic when both characters show up in the same session. I'd go withsomething else.

If you're a Bard or Wizard, you can say you were out studying - songs and folklore of the area for Bards, arcane lore for Wizards.
"And learning a new song was more important than aiding us in the fight against the orc tribe!?"
"Of course it is! I had to learn it before the granny passes away! If I hadn't done the job, the song would have been lost forever!"

Clerics, Druids, Paladins and Warlocks can say their Gods/Patrons called upon them and they were doing some minor task.
"Where had you been?"
"Hell. It was terrible."
"A place like hell?"
"No, literal hell!"

A Barbarian or Fighter might have been arrested in a bar fight.

A rogue might have been busy hiding himself from town guards after stealing something valuable or killing someone important.

Monks or Rangers might have been helping others of the same path to solve a dire problem.

Sorcerers might have been experiencing their power overflowing, becoming uncontrollable (easy to understand with Wild Magic Sorcs, but explaining all Sorcerers occasionally have these moments can be pretty believable), and decided it will be too dangerous to go on adventures with the others for fear of killing everyone by mistake.

You can think up lots of ideas based on fluff and character personality.