Charge and discharge LiPo batteries with BMS 3S
Without the manual for the BMS it is hard to give an authoritative response, but I really don't think that you can connect these devices in parallel.

A BMS typically want "single access" to the batteries in question to control current and/or voltage. One BMS does not know how much current to add to the flow if there is another one which it has no control over.

It could work if there was a secondary communication of some sort between the boards, but it appears to lack such a feature.