Hi, I have the structure below which, under solicitations, is bending a bit towards the direction of the red arrows.
Pipes are 40mm diameter.
I was thinking about moulding two half pipes composed mostly of several layers of unidirectional carbon tape and then glue with mma the half pipe to the existing pipes (along the orange lines).
The layup I am thinking is 10x layers of 50mm wide 650gm2 carbon unidirectional tape, alternated with some 50mm wide plain carbon tape.

What do you think about the idea and the layup schedule?

Also, maybe adding a 2 mm pvc core in between would help? I am just worried that the shear force could cause the two carbon frames to slip on the pvc core, so maybe a not cored layup would be better?

Yeh it looks like a terrible design for rigidity in that plane, it's got 0 bracing for it. Must be quite strong in the opposite direction! 

But yes, the solution to this is more about bracing the structure properly. It would be quite basic maths to work out what the approximate bending loads must be to be causing 40mm steel tubing to deflect, and then from there it won't be hard to work out that to make it sufficiently strong with carbon shells is extremely inefficient. What it would cost to make those shells vs what it would cost to weld in another steel bar, its a pretty obvious choice.