Multiple regression requires an assumption that the combination of the two simple regressions does not require. What is it, and what condition of the multiple regression does it affect? The multiple...

Multiple regression requires an assumption that the combination of the two simple regressions does not require. What is it, and what condition of the multiple regression does it affect?

The multiple regression of Salary on Years, Group, and the Years
 Group interaction (Table 5) reproduces the fits of the two simple regressions of Salary on Years for male and female managers. What’s the relationship between the residuals of this multiple regression and those of the two simple regressions?