Won't work.
1) you need a pull-up resistor as well.
2) you can't read the status of the pin that's connected to the interrupt only, and for proper quadrature encoder reading you need to know the status of both pins.
If you're suggesting to connect both the RPM and encoder to one interrupt pin, it's even worse:
3) when the encoder pin is low, you can't read RPM any more.
4) while the RPM signal is in its low state, you can't read the encoder.
(depending on the direction of the diodes the high/low may be reversed, this doesn't solve any of these issues).
Multiplexing pins can sometimes be done, but you have to be really careful about what you're doing on the same pin. On an ATtiny85 project I ran out of pins, needed an extra one, in the end used one pin as both a digital output and analog input. Works like a charm, but only because both parts are fully tolerant of the other: the digital output is for the EC pin of my EC sensor, the analog in on the same pin is an NTC. It's so far the only time I successfully multiplexed a single pin to handle two sensors.