It depends on however the IC of the power banks were designed from the beginning. Some IC are designed to only obsorb electricity from the input connectors (usually the small USB connector on the power bank and only export power to the output connector.) Some aren't which means that this kind of IC isn't secure unless it carries much smart management system.
They each are going to be fine, as charging different devices trough USB is their meant purpose. It does not matter if the other device may be a smart phone or an power bank. Just do not expect to total up the 2 charges. as an example if you chain a 4000mAh power bank with another 4000mAh power bank you will not have the equivalent of a 8000mAh battery, some current are going to be lost within the charging and also the actual charge are going to be slightly less.
If the input port is only for importing and also the output for exporting. Then you recognize the answer-One of the power bank can charge the other till the primary one runs out of power or the other absolutely fully charged.
Just confirm that:
1. they are of similar voltage
2. you use a short cable to optimize the transfer
3. you use the best output amp.
If they were of constant capability. Or, if the charging bank were of considerably larger capability than the bank being charged, it may absolutely charge it. The dc to dc converter within boosts the li-ion battery from 3.7 approximately volts to 5 volts, however it isn’t super efficient, in order that accounts for lot of the energy loss.