You can transfer epub files, wirelessly, from KOReader equipped device to KOReader equipped device, using the built-in "zsync" function. Once transferred, you can read it using the reader of your choice on the device. This is a one file at a time operation, so a cable approach might be preferred for transfers of a large number of files. Be aware though that Android and KOReader don't always work well together, I have to use an older version of KOReader (848) to have most things work on my Galaxy 7 Active.
Just tried "koreader-android-v2015.11-1174.apk" from
here, and it is working, so far.
Luck;
Ken