A pair of equations like this are called simultaneous equations - because you are trying to solve them both with the same values for \(x\) and \(y\).

title1 'Supply-Demand Model using General-form Equations'; proc model data=sashelp.citimon; endogenous eegp eec; exogenous exvus cciutc; parameters a1 a2 a3 b1 b2 ; label eegp = 'Gasoline Retail Price ...

For more information on these methods, see the references at the end of this chapter. There are two fundamental methods of estimation for simultaneous equations: least squares and maximum likelihood.