This would require calculating the horizontal shear flow of the composite section at the interface of the two separate pieces and designing the lag screws to resist that force. The shear flow and lag screw design is quite involved. Do you just want the answer or do you need calculations on paper, too?

There are two components and they are both quite involved. First is calculating the required horizontal shear flow. Second is designing the lag screw capacity to exceed the horizontal shear flow demand. The equations extensive and you should have an engineering background to apply them. The general procedure is above. 1. Calculate the horizontal shear flow demand based on the individual pieces and your design load. 2. Determine the required lag screw diameter and spacing for your individual condition to exceed the shear flow demand. If you would like all of the equations and an explanation for each, I can assemble this for you as an additional service.