A simple way to do this was to take the characters and join them as if you were multi-classing a single character. For example, let's say you had a lvl5 Trickster Rogue and a lvl5 Hunter Ranger.
- Pick one of the characters (on randomize it, or alternate it, or...) to be the base character, who absorbs the other.
- Assuming your picked the Rogue, start adding Ranger levels to that character using the same choices as the original Ranger did (Hunter specialization, spells (when applicable), etc)
- Remove HP (based on missing HP from original characers), spell slots, and add current conditions (poison, blind, fatigue,...) to the new character
In the end, you have a lvl10 character with 5 Rogue levels and 5 Ranger levels, and a pretty decent fusion of your players characters. This character (with both versions of the base character) should be created outside the game, so as to not delay the game a lot.
That being said, I find this homebrew to be a cool idea to do once, and that's about it. You will have 2 players wanting to control this character :
- They may disagree on what to do, how will they solve this problem? Roll against each other?
- Combat might be even more boring (they essentially will play every other turn in combat)
You will also have imbalanced characters, a lvl10 monster in the middle of a lvl5 party will outshine everyone else. Your other players might not enjoy this. It also means you have to tailor encounters for this specific character, and you'll essentially be creating bosses that the party could not defeat otherwise.
Erik: Actually, I think this merger won't make the combined character all that powerful; you don't have higher level spells than normally, your proficiency is probably only 1 point higher, your AC stays the same, but you lose half your actions.
András: Losing half your actions will make this character actually weaker than the 2 halfs alone. The biggest advantage is taking "half" damage from area spells compared to 2 characters.
Finally, multi-classing only works up to lvl20, so with higher-level characters, you'll need to homebrew even more stuff.
PS: This could be an interesting thought if you had a player join in the middle of the campaign wanting to play 2 separate characters, but with the option of joining them at will. So a party of lvl10 characters, and he would be playing two lvl5 characters, leveling one of them at a time. Maybe it could work, I've never done such a thing.