Hi Diego - I might not completely understand, but I think you should be able to complete your mirror there in one step if you switch to the front or right view and then pick the mirror axis along that diagonal, like this:



Sometimes it can help to switch to those other views - since the 3D view's construction plane is located on the XY plane you can get limited by some things there, for things that want to work in reference to the plane such as mirror.

Please let me know if I have misunderstood what you're building there.

But I've made a note to myself to look into adding some 3D mirror options, because I can certainly see that they would be useful in other situations. It will probably be a little while before I'll be able to get to them - it's not a high priority at the moment since doing a mirror + rotate axis is not too terrible of a work-around.

- Michael