Question: Color Coding does not work

How can I archive color coding? The two plots are colored the same, but I want them to be colored according to one common color scale, which means that each z value is assigned to a unique color for the 2 function plot.

f1 := proc (x, y) options operator, arrow; sin((1/2)*y+(1/2)*x) end proc;f2 := f1+2;
p1:=plot3d(f1, 0..10, 0..20, style=patch, color=proc(x,y) x*y end proc);p2:=plot3d(f2, 0..10, 0..20, style=patch, color=proc(x,y) x*y end proc);
plots[display](p1,p2);



thanks

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