An outer tubing hanger is secured to an outer string of tubing and landed in a wellhead housing. A Christmas tree is connected to the wellhead housing. While in a first mode, an isolation tube extends between the tree and the outer tubing hanger for conveying well fluid flowing up the outer string of tubing. The isolation tube is removed while in a second mode, and an inner tubing hanger secured to an inner string of tubing lands in the outer tubing hanger production passage. An upper portion of the inner tubing hanger extends into sealing engagement with the tree production passage for flowing well fluid up the inner string of tubing while in the second mode.