Hi Lara - boolean union is the first step, that will intersect the spheres and make an object that has edges where the pieces intersect with each other. Then you would use the Fillet command to put in smooth connections. I have attached a 3DM model file result of doing that. But you'll only be able to fillet up to a max radius size of 3 units, any more than that and the fillet would need to cross over this other nearby edge which MoI's fillet engine is not good at handling:



- Michael