Honestly, I think you should work on 1 game until its finish before go for another. I feel like its a year to make 1 update for each game, for just a short time of gameplay. The last time i played Serenity2 feels like it has 2 years ago. Like H00DEDRIOT says in the comment below, i also having trouble remember the story for Serenity, i will probably have to replay again the season 1 and 2, but i dont wanna do that every time you go work on the last human and we have wait so long for the next update, that kinda puts you off. Having to do that waiting between games is worrisome. At least for me, my biggest worry is that when a game takes to much time to be made, increases the chances of be abandoned or be put on hold for too long.
Working on a game takes a lot of time. Both games have great storys, but i feel like Serenity will go on for quite some time and since it has 2 seasons, probably(also hoping) it will have more in the future, you could work on last human untill its done. And then focus on Serenity for good.
If last human have season 2 that would be great too, but i would like to see the end of Serenity before start a new story.
;)