Question: How can I fit two or more curve (functions)?

A method of fitting a curve to data points so as to minimize the sum of the squares of the distances of the points from the curve.
For example (yei,x1i,x2i,x3i,x4i) is experimental data and relevant function is y=f (x1,x2,x3,x4), some unnkown parameters

(a1,a2,a3,a4) can be solved by minimizing the sum of squares of residues.
Procedure is:
ry:=(yei-y)^2;
Tr:=sum(f, i = k .. n);
da1:=diff(Tr, a1);da2:=diff(Tr, a2);da3:=diff(Tr, a3);da4:=diff(Tr, a4);
sam:=solve({da1,da2,da3,da4},[a1,a2,a3,a4]);
Last, the value of a1,a2...are obtained.

But, now, there are some experimental data, both  (yei,x1i,x2i,x3i,x4i) and  (zei,x1i,x2i,x3i,x4i). these two sets of data are in

conformity with the two funciton: y=f1 (x1,x2,x3,x4) and z=f2 (x1,x2,x3,x4) respectively. The trouble is that the same parameters

(a1,a2,a3,a4) is the two functions.

so, now how can I fit two curves synchronously and obtained a set of parameters which meet the two functions.
Can you give me some idea for simulating several multivariate function (curves) synchronously!
Thanks!!