A little while ago I wrote a little bit about the benefits, draw backs and difficulties of having large group of player characters (even possibly giving players more than one character). However, there is the other extreme as well. Having fewer player characters provides its own opportunities and drawbacks and I hope to talk about a few of those as well as a few solutions.
I Don't Want to Play Multiple Characters
Some people just prefer to play one character and get immersed in that role. In those kinds of situations, though there may be benefits to having a player play more than one character, it would detract from the player's experience. In these kinds of cases, there are other ways to still give some of the benefits of multiple characters while also respecting the player's wish to play one character.
A More Heroic Feeling
There are many people who would say that fighting a group of 12 undead feels a lot more heroic if there are 2 players than if there were a full party of 4 (or 8, 2 for each player). Honestly, I'd agree with them in general principle (the types of undead matter too).
How to Implement
We have two characters. No problem, we can just scale the adventure to the same difficulty but to two players. On paper this sounds fine and will provide players with a challenge. However, it will also be less dynamic of an encounter (there are less enemies) and the Dungeon Master will need to do some weird things to certain encounters, especially when facing a boss like encounter (the encounter makes sense with a dragon so we can't easily swap it out but the players are too weak and as a result the Dungeon Master will be forced to modify the stats). This will obviously not be a problem if you are making the adventure from scratch yourself. Even the problem of less dynamic encounters can be solved by having higher level PCs though some concerns still exist (you can't through an army of skeletons against a cleric who was turn undead that destroys undead and there will be holes in party roles).
There's an obvious solution to the problems of scaling. If you only have 2 players and they wish to tackle an adventure meant for 4 characters, they can play the adventure with characters that are higher level. Doing this can provide a great sense of power for the players while at the same time the large amounts of enemies makes death a real possibility. The problem with this kind of approach is that it takes quite a bit of skill to pull off. The math for doing this is usually provided in the rule books (compare difficulty at suggested level to charts for higher level characters), however, a party of 2 characters will have holes in their abilities compared to a full party. They may lack healing magic. They may lack arcane magic. They may lack a stealthy guy. They may lack a heavily armored bag of hit points. As a result, some adjusting on the fly may be needed. When done from scratch
Two Characters in One
Up until now I've gone over solutions that work within the rules. However, there's another solution. A player character can be allowed to be 2 classes (in a classless system, do the equivalent of granting double health, roles, etc.) at once and level in both classes at once (at the Dungeon Master's discretion, they can be the same class twice). This way, they have the hit points of two characters. You can also give the character two actions, two reactions and two bonus actions (I leave it at 1 move action because otherwise players are as fast as warhorses). I'd also suggest tracking actions per class (mainly to prevent hilarious action surge and spell combos). For role-play, it counts as one character. In combat, it's essentially two characters grafted together (when thinking about the rules, it should be considered this way).
Naturally, doing this has some major concerns. Synergies that were never possible before suddenly become possible. Can a player take the fighter class twice (if so, can they action surge twice in one “turn”, since now they get two turns interlaced together)? Do you let them take the same fighting style twice (treating it as two characters grafted together, even though this allows for a higher AC?)? Doing this essentially creates a brand new rule system that the Dungeon Master now has to rule on (not being a published, ruling will need to happen more often). If done correctly it allows the Dungeon Master to run a published adventure as written (for a few sessions I tried running this kind of monstrosity and played in such house rule systems before with great). It also gives the players a general feeling of tremendous power.
There we have the reasons for having smaller parties and some ways to implement them in role-playing, but specifically D&D 5th edition. The solution under “Two Characters in One” is something I'd only recommend experienced Dungeon Masters or Dungeon Masters who are ready to essentially make their own system attempt. However, there is still much fun to be had when playing with a smaller party. As always, feel free to comment and I'd especially like to hear other solutions and suggestions for this kind of situation.