Why did I burn my L9110s H bridge
Unconnected inputs on CMOS devices are bad; they cause the internal transistors to be neither on nor off. It's possible that this caused the top and bottom of the H-bridge to be on at the same time, directly shorting through the device. This would explain why both chips failed when you only provided one input to one chip. You need to tie the unused inputs off to GND.

It's also possible that the motor draws more than 750ma - you should connect it up to the power supply via a multimeter in Amps mode to check this.

The solder melts at about 300C. Presumably the chips simply got this hot. I know it's cheap hardware but I very much doubt it was any kind of manufacturing fault.

It might be better to start with a current-limited power supply: e.g. 500ma USB charger. Run it for a few seconds and see if anything heats up unacceptably. It should at least take longer to self-destruct at reduced power.