In principle, you do not require a decoupling membrane. You will just need to ensure that you have an expansion joint across the point where the two substrates meet.
However, things to consider are:
How stable is the ply floor? i.e. thickness, deflection etc..
Are you putting insulation boards under the UFH? (I presume it is electric UFH)
Do you intend to apply a floor smoothing compound over the UFH?