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Is the fact that you're interested in symmetric polynomials relevant? For polynomial rings in general defining the homomorphism is quite straightforward, so I would expect that by restricting the map you'll also get it for symmetric polynomials:
sage: R.<x0,x1,y0,y1>=QQ[]
sage: S.<X0,X1>=LaurentPolynomialRing(QQ)
sage: H=Hom(R,S)
sage: m=H([X0,X1,X0^(-1),X1^(-1)])
sage: m(x0+2*x1+3*y0+4*y1)
X0 + 2*X1 + 4*X1^-1 + 3*X0^-1
