  Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal
and/or collisional debris ejected from more massive interacting galaxies. We
use CO($1-0$) observations from ALMA and integral-field spectroscopy from MUSE
to study molecular and ionized gas in three TDGs: two around the collisional
galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H$\alpha$
emission is more compact than the HI emission and displaced from the HI
dynamical center, so these gas phases cannot be used to study the internal
dynamics of TDGs. We use CO, HI, and H$\alpha$ data to measure the surface
densities of molecular gas ($\Sigma_{\rm mol}$), atomic gas ($\Sigma_{\rm
atom}$) and star-formation rate ($\Sigma_{\rm SFR}$), respectively. We confirm
that TDGs follow the same spatially integrated $\Sigma_{\rm SFR}-\Sigma_{\rm
gas}$ relation of regular galaxies, where $\Sigma_{\rm gas} = \Sigma_{\rm mol}
+ \Sigma_{\rm atom}$, even though they are HI dominated. We find a more complex
behaviour in terms of the spatially resolved $\Sigma_{\rm SFR}-\Sigma_{\rm
mol}$ relation on sub-kpc scales. The majority ($\sim$60$\%$) of SF regions in
TDGs lie on the same $\Sigma_{\rm SFR}-\Sigma_{\rm mol}$ relation of normal
spiral galaxies but show a higher dispersion around the mean. The remaining
fraction of SF regions ($\sim$40$\%$) lie in the starburst region and are
associated with the formation of massive super star clusters, as shown by
Hubble Space Telescope images. We conclude that the local SF activity in TDGs
proceeds in a hybrid fashion, with some regions comparable to normal spiral
galaxies and others to extreme starbursts.
