No. You cannot use 2 4G SIM cards at the same time in Nokia 1. One SIM card will work on the 4G network and another will be on the 2G/3G network.
Yes, Nokia 1 does have the FM radio.
You can record 720p resolution videos from Nokia 1.
No. Nokia 1 does not have the front camera flash.
The Nokia 1 is an inexpensive Android smartphone, and as you can tell from the Nokia 1 review, you very much get what you pay for.