How do I add two R^2 values from a linear regression?

I sampled a molecule and fit the data to a linear regression. The slope of the regression is what I am looking for. I ran the experiment twice, so I have two different values for the slope, which I now want to average. How can I report my error in the averaged value? Is it appropriate to do it in terms of R^2? If so, how? Thank you very much!

I suppose I'll add another question while I am at it. Since I ran the two experiments separately and using different input (independent variable) values, I cannot simply combine all of my data points from both experiments into one data set and do a linear regression on that, right?

Is there a preference for either the first approach (averaging) or the second approach (combining the data together)?

If any of these questions may be answered it would be a huge help!