lack of players or intresting new idea.

[Sariel Vailo]

in truth its something i discovered form an earlier eddition of dnd and am testing in 5e. the gag as it is well it's silly but here goes. i allow each player to have trained up in two classes and continue to grow in both classes if say i dont have 4 players.
they can not multiclass they gain only halve the health and take the hd from the lower class. thus offering double the power half the health and three times more intrest or wackiness depending on class, homebrew dm and otherwise can be serious or deathly comedic

[Angelmaker]

Sounds like gestalt rules. Running a 5e eberron campaign with gestalt myself because i only had three players when we started.

- characters are not twice as powerfuul - action ecnomy still only allows you to do one action. Especially if you only allow the lower hitdice to count for playes, you will encourage two full casters or two martial classes tacked on to a chassis. Having players have two full casteing classes is a nightmare in terms of spell selection and can slow the game down considerably.
- saves can become quite easy with 4 save proficiencies
- design more powerful encounters, one ore two challanege levels above whats usually expected. Players get a lot of options with two claases, so the usual 6 encounters and two short rest design paradigma may not work for your group ( spoiler: it doesn't work in mine. ) with the lower hitdice rule in place, it becomes even harder to design challenging encounters for gestalt without bursting them down.

Just a few observations on my part. Keep on trucking and enjoy. :)

[DivisibleByZero]

Why not just have have each player control multiple characters instead of turning the game into an headache for yourself?

[Sariel Vailo]

So for caster issues ek plus options equal power