that's another way to do it
that way is not hard to do since you don't have to take the battery's much apart.
BUT both battery's are connected AFTER the protected circuits and this can potentially cause trouble in the long run.

And yes there is a lot of space in the mugen cover.
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I will be doing a tutorial in the near future when i make another dual battery with the battery's 'oig.buy'.
This will be picture by picture and description along the way.

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Another battery tested and added to post #1