I don't think I would just sum the two shearwalls because they are taking different unit shears. You have to assign the lateral loads based on the trib area for each on the right side. Once you have the load in each wall make sure that each capacity is adequate.
I would probably put a collector element in line with the inward shear wall on the right side and analyze it as two sub-diaphragms. The far right wall will not see much shear due to your flexible diaphragm action.
dgkhan - In a wood framed structure, a common collector element would be a typical floor framing member that is designed for the axial load induced by the unit shear in the collector element. Check out Breyer's book for some good examples, but you can calculate the tensile force in the collector by applying the unit shear multiplied by the length of the collector. Remember, you will be dragging the load from both diaphragms through the same collector element.
I would agree that using a drag strut is the best option. If the joists or trusses are in line with the wall I would then attach them to the wall using a combination of strap and blocking or a angle clip (such as Simpson A35's).
If you need both shear walls to resist a large load, then I would design the corner as a re-entrant corner (that is, make a drag strut both directions at the corner, using blocking if you have to) and apply the shear load as if there was one wall line (i.e. proportional to the shear wall lengths).
