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We present the first measurement of the $B^0\to X_{u}^- \ell^+ {\nu_\ell}~$
partial branching fraction in the end-point region of the lepton momentum spectrum,
above the threshold for $B\to X_c\ell {\nu_\ell}$ decays.
The analysis is based on a sample of 383 million $\Upsilon(4S)$ decays
into $B{\overline B}$ pairs collected with the {\sc BaBar} detector at the PEP-II $e^+e^-$
storage rings. We select $B^0{\overline B}^0$ events by partially reconstructing one $\B$ meson
via the ${\overline B}^0\rightarrow D^{*+} \ell^- \bar{\nu}_\ell$ decays then select
$B^0\to X_{u}^- \ell^+ {\nu_\ell}~$ decays
identifying a second high momentum lepton. In the momentum interval
ranging from $2.3$ to $2.6$~GeV/c we measure the partial branching fraction
$\Delta{\cal B}(B^0\to X_u\ell\nu)=(1.30\pm0.21_{stat}\pm0.07_{syst})\times 10^{-4}$
where the first error is statistical and the second is systematic. By comparing
this measurement with the one obtained from untagged $B$ decays we obtain
$R^{+/0}=\Delta{\cal B}(B^0\to X_u\ell\nu)/\Delta{\cal B}(B^+\to X_u\ell\nu)=
1.18 \pm 0.35_{stat} \pm 0.17_{syst}$. Using this measurement we extract
a limit on the contributions from processes breaking isospin symmetry
in charmless semileptonic $B$ decays.
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