My girlfriend and I had this issue, we found a post (I'll update with a link if I find it) that helped us solve it and has been working every time we do it. 1- Log in normally and try to join each other 2- Once you get the "Unable to join" message, you both double tap the home button 3- Go to Settings > Network > Go Offline (on both consoles without closing out of Destiny) 4- Wait 15-30 seconds then click Go Online 5- Return to Destiny and try to log in again and join each other 6- If it still doesn't work, try doing the same, but this time close the game AFTER going offline/online. It seems to be random on which way will work, but the most times I've had to attempt this has been 5-6 times and it works within 10 minutes at worst. Notice, if one of you gets error code Canary over and over, you'll have to hard reset and power cycle that console. Hope this helps! Let us know! Feel free to pm if need more help

Hey man, I hope my post will help you. So my roommate and I play on the same connection. When it first started our connection would not work at all, we resorted to having him use Xfinity or guest wifi and me on the home network. This worked for about a year and a half but it turned to crap when I was ddod and had to get the internet company to come fix my connection. They did and then my roommate couldn't log on to the guest wifi so we were forced to play on the same connection. What I was able to do was me connect via Ethernet and him with wifi. It didn't work at first but we made it work by unplugging our ps4s and giving them 5 minutes chilling and then we connected them and everything worked well after that

Could be possible that they haven't been able to replicate the issue on their system. I have friends in play with that play with their wives/family every day and have never had an issue.