TL;DR
A 2 PC party does not function like just half of a 4 PC party! More to it than that.
Combat
A current party of 2 acts very differently from 4 or 6. (DnD5e)
My original assumption was that if 4 goblins would be a challenge for 4 1st level PCs, then 2 goblins would be a challenge for 2 1stlevel PCs. They are not. 2 goblins will get roasted in no time.
Conversely, if a single CR1 critter is a medium challenge for 4 1st level PCs, that same creature is likely to absolutely roast a party of 2. There seems to be so much fucus, specifically in dnd 5e on party dynamics and action economy, that a reduction on that level wrecks the match in a pretty major way.
***** I am super curious to try a more narrative dice game like FFG SW to see if a party of 2 holds up better there. I suspect it will.
Plot
The pace and focus of the game will also be quite different. So far, a party of only 2 is just chewing through my plot bits, as well as making massive progress on their individual backstories. I did not “plan” anything past 5th level, but here they are late into only second level and they have ground through 75% of my expected plot chuck dispersion schedule already. Plus… PLUS both have made good strides on their character backstories, which I’d originally intended to carry the game past 5th level if it was still going strong when we got there, but they might have their character arcs nearly resolved by that point.
To be clear, this is a PC party of an intermediate player, and a total newb who still has to ask which die is the d12. It’s not like they are two old school sages just chewing up my adventure hooks like goldfish crackers.
Speed
My game before this was, at one point, a party of 8 PCs plus 1-3 “guest” players that cycled in and out. I would barely get 3 encounters in for a 3 hr game let alone touch 8-11 backstories even in a superficial way. Now with 2 PCs, I can do 3 full combat encounters, 2 role play bits, plus 2 vignettes, and still have the time to do 1 secret drop per PC backstory in a 3 hr game. If I have 2 monsters and they both miss an attack, we can bang out a whole round in less than a minute. I have a unique point of comparison because 2 of my 6 4e players were pharmacists who over-analyzed everything and a full round could take 15 minutes. Like watching damn paint dry sometimes. Now, I find myself stunned that it’s the baddies turn again, already, as we just grind up monsters like a chipper shredder.
Sum Up
The saving throw that will keep this game interesting is that the PCs are stuck in between 2 rival BBEGs who have both concocted overly complicated schemes to ruin Waterdeep in their own special way, so it is very unlikely I will run out of plot. I will say, there have been 2 sessions where they took someone along with them, not exactly a DMNPC, but they did get combat help from a person with plot armor, and it felt extra cheaty-deusexmachina, loss of agency, etc. That was the closest they came to feeling railroaded, or like bystanders, even though they were still 2/3 of the party. I am going to try a combat-buff NPC again this weekend, but give the NPCs agency to the players, and see how that works.
The main bit I want to impart to anyone who runs a “Holmes and Watson” style 2 PC murder mystery game is to watch out for area attacks, bursts and blasts etc. If both PCs get caught in a fireball, one attack could land a TPK. It’s happened twice already in 6 game sessions. Also, traps and environmental damage mean so much more when there is less of a total party hit point pool to go around.