Installing a y pipe

I had sweating ducts.i pulled the drywall down and wrapped all the ductwork.the main stack, two 4 inch ducts and two 6 inch ducts in my dining room and into the kitchen. I got to one spot where the two 6 inch ducts go through the main truss in the center of the house. The builder put a third truss butt up against the main support truss but they did not line up the openings. They squished the two 6 inch ducts down to about 2 inches to get through the opening. I don’t have anywhere I can rerun this. Can I put put a Y pipe on the two 6 inch ducts to reduce it to one 6 inch pipe then put another Y pipe on the other side to go back to 2 ducts?