Elden Ring Nightreign is a cooperative spinoff with an asterisk. You can play it with a group of three or by yourself, but not in a duo, according to an IGN interview with director Junya Ishizaki.
As someone who played through the game using the Seamless co-op PC mod, I wouldn’t be surprised if a modder figures out how to make two-player co-op work. But until then, the rules are the rules.
To play online co-op in Lego Horizon Adventures, pause the game and then select the 'Invite ... Nintendo Switch, or PC, you'll only be able connect with friends who are also using the same format ...