If you want to use v4l2 source instead of uridecodebin to implement multiple sources, you need to use the pipeline like below. You can refer to the deepstream_test_1_usb to create multiple branch before the nvstreammux.
v4l2src1->caps_v4l2src1->vidconvsrc1->nvvidconvsrc1->caps_vidconvsrc1->
nvstreammux->........
v4l2src2->caps_v4l2src2->vidconvsrc2->nvvidconvsrc2->caps_vidconvsrc2->