Play videos from another android device over wifi
Run an FTP or HTTP server on your Android device which has bigger storage. There are many apps on Play Store which can create a server on Android like FTP Server Ultimate or Servers Ultimate Pro.  Creating a server on them is very easy. You can get an idea from here as well (see #Method 3).

Install a file manager which allows access to remote storage such as ES File Explorer or MiXplorer. Both are very intuitive and can create an FTP server or either access it. (See step 5 here for former's usage.)
It goes without saying that both devices need to be connected under same Wi-Fi or hotspot.

Install a multimedia player such as MX Player to stream videos.


That's it.