Hi,
as both TP4056 migth not have the same caracteristics or they differ because of different oparating temperatures you can not expect same "output voltages" during charging process, for the constant voltage period as well as for the constant current period (the voltage drop across the current source might differ). Consequently there might be current flowing from one TP4056 into the output of the other one. To avoid this the use of two diodes might seem to be a "good" choice to protect the individual outputs (anode @ out+, cathode @ bat+). Unfortunately the voltage drop across these diodes will reduce your effective constant current by about ~0.3 V (zener diodes). Ideal diods migth do the job to minimize the voltage drop and to minimize power losses.
BR