STUPID, DANGEROUS IDEA!!!
Do NOT do this.
OR, if you do, please inform us where we can get in on a pool for your fire and life insurance.
If you want the electrical info, lets see, the two batteries will equalize at an in-between voltage, dependent on cell capacities, in an exceptionally short period of time, wires may melt, lower voltage battery will likely blow the BMS, but may overcharge cells, they may explode or catch on fire, starting a thermal runaway where adjacent cells overheat and catch on fire, etc.
This could result in cells from the higher voltage battery being dangerously Undercharged, with similar results to above. less likely, but still possible. blowing the second BMS also a possibility.
The least damaging result would be one blown BMS.
As for most damaging, there are dead people who have tried this. Houses burned down. Livestock slaughtered.
OK, not that last one.