What you’re talking about here is a slice-to-volume reconstruction. There’s no facilities to do this kind of super-resolution within MRtrix3 (yet). @bjeurissen or @dchristiaens may have their own code to do this, but it’s much more complicated than simply merging the information.
That said, you can probably merge these datasets into one if you think this is something you want to try, but you’d need to regrid each one onto the same higher resolution 2x2x2mm grid, maybe using mrresize to provide a template image at 2x2x2mm resolution (or an even smaller voxel size), and mrtransform to regrid both dataset onto the same template before mrcat – but there’s no guarantee the two datasets will be aligned, and simply merging will not super-resolve as you might hope. It’ll probably give you some resolution enhancement, since even humble linear interpolation has been shown to do that with DTI – but it’s not true super-resolution as you would expect from dedicated slice-to-volume methods.