Here I answer your questions:
a) forward to email any SMS the same as SIM1?
SIM2 can perform the same function of forwarding to email any SMS. Only this will be done with the number assigned to SIM2.
b) Will it use the same config set up for SIM1 on the router pages?
No, if you notice there is a configuration menu for each SIM. In Network→Mobile→General you can see the configuration of SIM1 and SIM2. That is, you can configure each SIM separately depending on the provider.
c) What data will it use to do it, the current active SIM1 or the SIM the SMS belongs to?
As mentioned above. You will find a knob for Default SIM in the mobile configuration menu. So you can set SIM 1 as default and SIM 2 as a backup.
Note that the RUTX09 has only one modem. So the two SIMs will not work at the same time. Here is a link on how to configure and information about the Mobile configuration.
In case you want to use SIM2 as a backup. You can set up a failover. Here is a link on how to configure it.