Do NRF24L01 modules have to use the same SCK-Frequency to work together?
They don't have to use the same frequencies. SCK frequency determines how fast the master (your Atmega 328p) can read/write from the nRF24. It does not determine how the nRF is configured or what is the over-the-air speed.

It may make a difference only if your are sending LOTS of data from a master that uses faster SCK and the master with slower SCK just can't read it fast enough. A logic analyzer (even the $8 kind) helps a lot with SPI debugging.