Would it be safe to use two 9 V LiPo batteries in parallel to power an Arduino Uno?
The short answer is NO, YOU CANNOT PARALLEL THE BATTERIES
In general paralleling LiPo batteries can be done but the "batteries" you point to are NOT good candidates. They are in fact NOT 9V batteries at all, they are single cell LiPos (4.3V max) with a DC-DC convertor producing a regulated 9V output.
Paralleling DC-DC convertors is bad news unless specifically designed to do so.
If you want to use more than one battery to power your Arduino you would need either:
- Some power switching design to use first one then the other battery.
- Some form of linear regulation that would allow current to be draw from two batteries.
As an example of what might be possible you could simply diode OR the two 'batteries' together using Schottcky diodes . This would stop the DC-DC convertors interacting. The two batteries WILL have different output voltage even if only a few mV. The diodes would ensure that the highest voltage 'battery' supplies the load. As it discharges then eventually there would be some sharing of current from each battery, though it would not be equal balancing.
If all you are powering is the Arduino this may be fine. It is unlikely to be suitable where you may be running motors or higher load current where you'd want an even balance between the two 'batteries'.