After a disappointing first 2p experience where I was stuck with a huge pile of gems, unable to build anything, I decided to try and come up with a variant that would make Charon Inc feel more like the game with more players. Here is that variant.
There are only a handful of small changes to the base game:
1. 5 cards are dealt to each player at the start of the game, 4 in each following round. Each round, players discard two cards to the common pool but may still only build one building from the common discard each round.
2. Each player may build two of each numbered building (of the 5 available in the deck). They only receive a replacement card if dealt something they’ve built two of.
3. Each player places 5 flags, the extra is the one in front of them. In a two-player game, it not unreasonable for at least one player to remember one of the colours, and the extra tension on the board is necessary. I would suggest placing the non-special flag first.
With these there's more tension around grabbing crystals, the special powers become super important, and people have to make hard choices about how to most effectively build buildings with their gems, even considering whether to deliberately build doubles to increase the chance of a lucky draw from the discards.
I've posted a session report with the variant here, with more details on the problems I'd encountered, and illustrating how the game flows and solved those when played with the variant. I'm pretty happy with the 2p game now.