For actual files and new files you need to set acl with:
setfacl -Rm d:u:otheruser:rwX,u:otheruser:rwX /your/dir
use X (uppercase) if you want only directories to be x and not files. Use x (lowercase) to include files.
d:u:otheruser part is for default (new files) and the 2nd part of the command is for existent files. You can set different permissions on each part if you prefer.
-R is for recursive
Thanks to user390689 (original asker) input, mask (and default mask) needs to be added too because
setfacl should (according to man setfacl and my own experience) create one based on the union of the existing groups acls and the added permissions. As in this case you are supplying only user and no groups, there is no acl for the groups and the union is empty so you need to do it manually with:
setfacl -Rm d:m:otheruser:rwX,m:otheruser:rwX /your/dir