2 players in household, one lags

[Tricsun]

So having a strange issue.  I have 2 people on my home network that play the game.  When either one of us are the only ones on, there's no issues, however, as soon as we're both on, only their connection starts to have bad lag spikes, and i have no issues at all.  Tried searching around the net a bit to see if there's something out there, but nothing that seems like it fits.