Can I use multiple voltage regulators in a circuit safely?
No, this is not a good idea.

Check the datasheet of the regulators, but most don't like reverse voltage.

Parallel voltage regulators can't be counted on to share current.  One will always have a little higher setpoint than the other.  This could also lead to instability, depending on the nature of the controllers in the regulators.

A better way to solve this is to combine the two voltage sources before a single regulator.  At this low voltage, you can use Schottky diodes.  Put one diode in series with each voltage source.  Power will then automatically be taken from the higher of the two voltage sources.  Make sure to put something like a ceramic cap after the diodes physically close to the input of the regulator.

You can still use multiple regulators to spread the dissipation and to reduce voltage drops to distant parts of the circuit.  You bus around the higher voltage out of the diodes, then regulate that locally as needed.  However, you don't tie the outputs of multiple regulators together.  You have each power a different part of the circuit instead.

If you want to minimize local dissipation, you use a buck switcher after the diodes.  This makes a little more than the minimum input voltage of the regulators.  You filter that a little and bus that around.  Then you make the regulated voltage from that as needed locally.  For example, if using a 5 V LDO that requires 5.5 V in, you might bus around 6 V.  Each local regulator would then be 83% efficient.