No I guess it's how the games are working, @Stoked4Good, unfortunately.
I have the same problem with Mortal Kombat 11. When you play local coop only one account can be logged in, the second one can join though with the second controller and then we can play together, but only the person with the account logged in (in this case controller 1, because he started the game) get the ingame-rewards while playing. Even though the second person has another Stadia account, we didn't manage to add them both to the game, because there is no option at all to do so.
Talked even with the support yesterday, but they mentioned as well, that it's apparently only working for Mortal Kombat 11 if we would do the following: The second Stadia account has to buy Mortal Kombat 11, then the first account can start an Online-battle-mode and invite the second Stadia account to it. Then each could would be able to earn the own rewards, but they are playing on two different devices then.
Guess for the other games it might be same, haven't tried it in Darksiders Genesis, but as you mentioned it's also not possible to play split-screen with a second account. ![]()
I still feel this is not really well implemented or thought about, because my BF even told me that Xbox 360 for example allowed players in local coop to login with their account when activating the second controller, so that you can play a game owned by "the hoster" and the second one can just login with his account and enjoy and also earn rewards. So I wonder why this is not available for Stadia in local coop?
They better fix/implement this stat. As that's how most consoles operate, with the ability to login with your own profile. Even if no progress on both accounts, as some games can limit this, at least for the sake of unlocking rewards as you mentioned, and personal customizations etc. I guess B3 will be the true test. If they don't pull this off, as expected, I might have to pull the plug on my 110% Stadia commitment, unfortunately. Just when I was ready to leave PS behind. My PS Plus sub expires this month too.
I think that this problem would be automatically solved when they will release Stream connect, Given that it should be as playing a game in the net but in split screen.