Wawa:
Are you sure the modules don't mind being so close to each other.There are some counterfeit HC-12 modules around that work ok with another counterfeit one, but not with an official one. The printing on the crystals must be exacly the same for a pair to reliable work.
Leo..
Both hc12 in the testing jig perform 100% in all baud rates, as well close together or spaced metres apart.