  The main purpose of this work is to investigate the properties of the
non-thermal emission in the interacting clusters pairs Abell 0399-Abell 0401
and Abell 21-PSZ2 G114.9, found in an interacting state. In both cases their
connection along a filament is supported by SZ effect detected by the Planck
satellite and, in the special case of Abell 0399-Abell 0401, the presence of a
radio bridge has been already confirmed by LOFAR observations at 140MHz. Here,
we analyse new high sensitivity wideband (250-500MHz) uGMRT data of these two
systems and describe an injection procedure to place limits on the spectrum of
Abell 0399-Abell 0401 and on the radio emission between Abell 21-PSZ2 G114.9.
For the A399-A401 pair, we are able to constrain the steep spectral index of
the bridge emission to be alpha>2.2 with a 95% confidence level between 140MHz
and 400MHz. For the A21-PSZ2 G114.9 pair, we are able to place an upper limit
on the flux density of the bridge emission with two different methods, finding
at the central frequency of 383MHz a conservative value of fu_1<260mJy at 95%
confidence level, and a lower value of fu_2<125mJy at 80% confidence level,
based on visual inspection and a morphological criterion. Our work provides a
constraint on the spectrum in the bridge A399-A401 which disfavours
shock-acceleration as the main mechanism for the radio emission.
