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You can perform such substitution by explicitly going over the coefficients of x and y:

sum( cy*x^(dx-min(dx,dy))*y^(dy-min(dx,dy))*w^min(dx,dy) for cx,dx in P.coefficients(x) for cy,dy in cx.coefficients(y) )

You can perform such substitution by explicitly going over the coefficients of powers of x and y:

sum( cy*x^(dx-min(dx,dy))*y^(dy-min(dx,dy))*w^min(dx,dy) for cx,dx in P.coefficients(x) for cy,dy in cx.coefficients(y) )

You In symbolic ring, you can perform such substitution by explicitly going over the coefficients of powers of x and y:

sum( cy*x^(dx-min(dx,dy))*y^(dy-min(dx,dy))*w^min(dx,dy) for cx,dx in P.coefficients(x) for cy,dy in cx.coefficients(y) )