= Solution
Let $\Delta t=t_d-t_g$ and $\Delta\mathbf x=\mathbf x_d-\mathbf x_g$. A <spacelike separation> obeys
$$
\boxed{c^2\Delta t^2-|\Delta\mathbf x|^2<0.}
$$
Under a boost with <velocity> $\mathbf V$, the time separation is $\Delta t'=\Gamma(\Delta t-\mathbf V\cdot\Delta\mathbf x/c^2)$. Choose
$$
\boxed{\mathbf V=\frac{c^2\Delta t}{|\Delta\mathbf x|^2}\,\Delta\mathbf x.}
$$
Then $\Delta t'=0$, while $|\mathbf V|/c=c|\Delta t|/|\Delta\mathbf x|<1$ follows from the spacelike inequality. Therefore an inertial simultaneity frame exists. This choice has no transverse boost component and is not claimed to be the unique such frame. If the events are already simultaneous, $\mathbf V=0$ suffices.
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