Hi,
Running two 4K encoding on Jetson Nano may not achieve 30fps for the two encoding threads. We suggest downgrade the resolution. For plug-and-play cameras, you may consider to use Raspberry Pi camera V2, which is imx219.
We support hardware acceleration in gstreamer and jetson_multimedia_api. If you would like to quickly construct your use-case, may try gstreamer. Please check gstreamer user guide
If there is concern in burs bitrate, you may try to run in CBR mode and set virtual buffer size. This would balance frame size of P frames and does not see much size gap between I and P frames. Please take a look at:
Random blockiness in the picture RTSP server-client -Jetson TX2 - #5 by DaneLLL