Conjoined vases of this form decorated in underglaze blue are extremely rare. One identical example is known in the Shanghai Museum, illustrated by Liu Liang-yu, Ch'ing Official and Popular Wares, A Survey of Chinese Ceramics, vol. 5, Taipei, 1991, p. 151. Another Qianlong-marked vase of the same shape by covered with a Ge-type glaze was sold at Christie's London, 11 May 2005, lot 9.
The unusual shape seen on the present vase follows similar shapes from the Yongzheng period. Compare this to a Yongzheng mark and period Ru-type vase in the collection of the National Palace Museum, Taipei, included in the exhibition Qing Monochrome Porcelain, National Palace Museum, Taipei, 1981, no. 77. Also see a celadon-glazed vase from the J.M. Hu collection, sold at Sotheby's New York, 4 June 1985, lot 40; and a Guan-type vase from the James W. and Marilyn Alsdorf Collection, sold at Christie's Hong Kong, 23 March 1993, lot 735, and again on 2 November 1999, lot 524.