documentclass{article}%
usepackage[all,2cell,cmtip]{xy}
UseAllTwocells
begin{document}
[
vcenter{hbox{xymatrix@!0@=15mm{
tlar[r]^-{t}ar[d]_-{l}xtwocell[rd]{}<>{2}&trar[d]^-{r}\
blar[r]_-{b}&br
}}}
]
end{document}