concatenating columns from multiple unrelated 1-row resultsets
Two subqueries?

SELECT Q1.A, Q1.B, Q2.C, Q2.D
FROM (SELECT SUM(A) A, SUM(B) B FROM @X) Q1(A, B)
CROSS APPLY (SELECT SUM(C) C, SUM(D) D FROM @Y) Q2(C, D);