Google's Inbox app is a pretty great email client, but it has always lacked something that many mobile and desktop email applications have - a merged inbox view. You always had to check each account separately, which can take at least two taps to switch between. Thankfully, Google has finally added an 'All inboxes' view to the app.
Tapping the new option in the sidebar does exactly what you would expect - it shows all your messages from all your accounts. However, none of the actual sorting that Inbox does is visible here. It's just a straight list of all your recent emails. I really hope sorting/categorization is added to this screen soon, because right now it's not very helpful.
The feature should already be live for everyone, as long as you have the latest version of the Inbox app. You can download it from the Play Store below.