It should not be possible to assign the same metric value to two interfaces. If you attempt to set identical metrics, you should receive a pop-up window with the following message:
You can configure the mob1s2a1 interface with a metric of 1 and the mob1s1a1 interface with a metric of 2. In this configuration, your router will prioritize SIM2 if it is available and active; otherwise, it will automatically use data from SIM1.
Best regards,