What will happen if I solder two 120mm 12v computer fans to one 12v phone charger?
Take a look at the 12 volt power supply and it should have printed on it the specifications.  You already know that it provides 12 volts, but how many amps is it capable of?

Then take a look at your fans.  They should also have printed on them the voltage and required amperage to run.

If the combined amperage of the two fans is less than the amperage that the power supply is capable of supplying then you are OK to power the two fans via the one power supply.

Of course both fans must be 12 volt fans.

As far as RPM is concerned,  I don't know a lot about how fan speed is generated, but I believe that RPM is dependent on voltage and since the voltage will be the same for both fans, RPM won't be affected.

Good luck.