ATmega328P and other chip sharing clock
Sharing a crystal between multiple devices is a very bad idea. The crystal is only a piece of the oscillator, the rest of it being a small bit of circuitry within the IC itself. Having multiple ICs connected to the same crystal will cause them to fight over who is driving it.

The datasheet for the CH340T/R, which I was able to find in English, says the following:


  When CH340 chip is working normally, the outside must supply 12MHz clock signal to XI pin.


This seems to imply that you can feed an external 12MHz clock into the chip and it will work. So we take the clock from the '328P and pass it to the '340G.

Programming the CKOUT fuse on the '328P will cause the internal clock to be output on CLKO, which we then connect to XI of the '340G. This means that that you will lose PB0/D8, but no longer need a second crystal. This also means that you will be unable to use the prescaler since doing so will affect the clock output.