Router behind router - how to prevent second router clients from accessing first router clients

I would like to set up the following:
Internet -> modem -> router 1 (ASUS RT66U) -> router 2 (ASUS RT66U)

I will set up a different IP range for both routers:
Router 1 192.168.1.x
Router 2: 192.168.2.x

Router 1 clients - office equipment, VOIP etc
Router 2 clients - guests, using wifi

I read that router 1 clients (further R1C) will not have access to router 2 clients (further R2C) but R2C will have access to R1C if they know the IP address of the R1C.

Goal: To increase security, R2C...

Router behind router - how to prevent second router clients from accessing first router clients