Same exact problem me, my brother and a friend are having. Brother and me are on the same internet and can join our friend's server but kicks one of us if one is already in the server. We made it work by using a VPN but probably not ideal since it raises the ping of the VPN user but the silver lining is only one of you has to use the VPN. You can set the VPN to your own country to minimize the ping issue. Hope this helps!
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mabeek
@mabeek