Constrain 2 skeletons with different bone orientation without getting double transforms

On the left I have the the UE4 skeleton it has weird looking bone orientations but this is needed for it to work with the UE4 mannequin. Because a skeleton like this is hard to rig I want to create a skeleton with normal bone orientations (the one on the right) create a rig for it and than constrain the original on the left to the new one on the right.

But there is the problem I cant manage to constrain them properly. When I use child off constrain I get double transforms and when I use copy transform it is totally messed up no matter what space I use due to the different orientations. Does anyone know how I would do this? Thanks!

Here the .blend if someone wants to test :slight_smile: https://mega.nz/#!T1IlRCgb!zES1EacKalH_bwP6FLIl5hjx2v3MVX_RS3i7vDd5e9c (was not able to add it as attachment because this Account is new)

Duplicate the left (UE) armature. Join your duplicate to the right armature. Parent the bones from the duplicated armature to appropriate bones in the right (usable) armature. Copy transforms from duplicated bones to UE armature bones. Move the duplicated bones to a different bone layer, or hide them, whatever you want-- those bones aren’t for animating, they’re just to get appropriate axes for copy transforms.

This works like charm, thank you so much!