Still not a full answer, but maybe pointing towards the solution.
PiecewiseExpand[
FourierTransform[
Sinc[\[Omega]1Sinc[ω1 - \[Omega]2]ω2],
{\[Omega]1ω1, \[Omega]2ω2}, {t1, t2},
FourierParameters -> {1, -1}
],
t1 \[Element]∈ Reals
]
This does not work with Sinc[b (\[Omega]1ω1 - \[Omega]2ω2)]
, though and I don't know why. It should only be a matter of the transformation formula, but apparently, Mathematica is not clever enough to apply it automatically.