We calculate form-factor ratios between the semileptonic decays barB-textgreaterDtextasciicircum+elltextasciicircum-barnu and barB_s-textgreaterD_stextasciicircum+elltextasciicircum-barnu with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of the neutral B decays, which are needed for measurements of BR(Btextasciicircum0_s-textgreater mutextasciicircum+mutextasciicircum-). We use the MILC ensembles of gauge configurations with 2+1 flavors of sea quarks at two lattice spacings of approximately 0.12 fm and 0.09 fm. We use the model-independent z-parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are $f_0textasciicircum(s)(Mtextasciicircum2_pi)/f_0textasciicircum(d)(Mtextasciicircum2_K) =1.046(44)_stat.(15)_syst.$ and $f_0textasciicircum(s)(Mtextasciicircum2_pi)/f_0textasciicircum(d)(Mtextasciicircum2_pi)=1.054(47)_stat.(17)_syst.$. In contrast to a QCD sum-rule calculation, no significant departure from U-spin (dtextless-textgreaters) symmetry is observed.