Single ET340 Multiple Cerbo GX?

I am in a situation where I have two GX devices (responsible for different Battery banks + 3x Multiplus II in ESS mode)

But only a single Grid meter. Is it possible to connect two rs485 to USB to the output of the meter?

Not directly, you would need an rs 485 splitter for that or program something with node red

Any Splitter in particuar that would work?

I have no idea, but any good quality (industrial) splitter should work