  E-strings arise from M2 branes suspended between an M5 brane and an M9 plane.
In this paper we obtain explicit expressions for the elliptic genus of two
E-strings using a series of string dualities. Moreover we show how this can be
used to recover the elliptic genus of two $E_8\times E_8$ heterotic strings
using the Horava-Witten realization of heterotic strings in M-theory. This
involves highly non-trivial identities among Jacobi forms, and is remarkable in
light of the fact that E-strings are 'sticky' and form bound states whereas
heterotic strings do not form bound states.
