A better equivalent would be to say R1 carries the current for both LEDs, so R1 is equivalent to two 200ohm resistors in parallel, R1a and R1b.
Now D1 is fed effectively fed through R1a+R2, with a total value of 300ohms, and D2 fed through R1b+R3, also 300ohms. So the total equivalent resistance in series with each LED is 300ohms.
This is strictly true only when the LEDs are balanced. If they have slightly different voltages, the solution for individual and shared resistors will be slightly different.