First and foremost, I would fit the two models seperately because a and d are important in both model, but b and c are only important in fitting the data for y1. What is your goal for y1 and y2? Based on your profiler it looks like you are trying to maximize both values. You can save the fit models back to the data table and then make a combined profiler from there and then do the optimization.
Second, even with the most reduced model possible y2 has a lower R-square than you would like, but based on the data, that's the best you can do with the available inputs. Are there other possible inputs that could/would influence the value for y2, something not in your DOE? Is there any wiggle room to have a higher d value? If maximizing your outputs is your ultimate goal then a higher d value seems to be the way to go. You may want to augment your DOE with higher d values to see if this holds true for both y1 and y2. Having a higher d value may not be feasible, but it is at least worth looking at to see what is possible if your goal is to maximize y1 and y2.
HTH
Bill