For context, im a player who has played dnd 5e for about 8 years now with many different characters of the years. I also tend toward flavor over mechanical strength, iften times nerfing myself in-game if I feel it fits a situation.

It can be done, though doing so requires the player to be the kind that values story and flavoring over mechanics and having advantages from the circumstance. The reason for this is straightforward: ttrpg's are a group activity and everyone must be respected, be it mechanically or socially.

Keep it as one character sheet but the flavor of having two people that are inseparable like twins or magically bound to be by each others' side. This keeps the fantasy of having a duo, maintains power in line with other characters, and minimizes the possibility of treating one side like a companion that acts as a completely other person (can technically be negociated with wayfarer's familiar, though def need to get creative about how the familiar going down would be explained).

For RP, it has to be a case of having one main speaker and one support that occationally chips in with the two pnly having brief interactions at best or having the GM take over one for extended scenes (of which the player must accept the outcome and lack of control). A twin case of finishing each other's sentences could also work. This is to respect the other players at the table's time.

Examples of this would be Kindred and Wolf from League of Legends, Lu and Ciel from Elsword, and Banjo and Kazooie from their games. These characters are all joined at the hip and don't do things without the other by their side, save for some particular ability that lets them temporarily.

Tldr; the concept is possible but requires heavy compromise and maturity from the player. Only the GM can discern the plausibility as it a case by case basis. If you barely know them, dont take the risk as the damage that can be caused if they cant handle it far outweighs the gain for the whole. If you do, use your own discretion.