Wiring two LDOs in parallel with only one diode between
Electronic industry has a solution for this case. It is solved by a special class of devices called "power distribution switch", within PMIC group of IC. 
Search Digi-key for this type, "PMIC - Power Distribution Switches, Load Drivers", then select, say, "2:1" in the column "Ratio - Input:Output". 

Essentially these are switches based on FETs, as "ideal diodes". Here is an example, FPF1320UCX.