Hi and Welcome to the forum!
Live video streaming is always a question of getting the cleanest connection possible so that encoded video running over the top has the best possible journey ( ie shortest path, least amount of packet loss). However, how you tweak and adjust the things you can to fine tune that connection depends on your requirement.
I’ve worked with customers who need the most reliable video stream possible with the lowest latency (<100ms) and who use WAN smoothing to duplicate the video stream 4 times across all cellular WAN links to make that happen. The compromise is that a single 4Mbps Video stream consumes more than 16Mbps of cellular data.
I’ve also worked with customers who want the most bandwidth possible and don’t care about latency. So we add network buffers everywhere (increasing latency to more than 2 secs), turn on Forward Error correction, and up the the bitrate on the encoder. I’ve done 100Mbps streams over cellular reliably between countries this way for live remotely produced studio shows.
What type of live stream are you doing? Is high bandwidth more important than low latency?
It sounds like you are getting congestion on your cellular links when you combine 2 or more. The first thing I would try is Dynamic Weighted Bonding as that is proving very successful for me in multi-cellular deployments. Then enable WAN smoothing if DWB is not enough.
Good luck!