If things were working fine and now are not, then something has changed on your end, your ISP's end, or somewhere along the connection. You could try to remedy the situation by renaming one of your PS4s if you've not done so, but since things used to work and now they don't, that probably won't fix it. Start by double checking to make sure your ISP isn't throttling you, as reduced speeds can cause a problem (yes, Comcast has data limits and will throttle you if you hit them). If they're not throttling you, then your connection isn't quite stable enough by the time you add a second console playing Destiny. Fast internet doesn't mean good internet. Contact your ISP about doing a line test while you're both attempting to play so they can see if they can detect an issue with your connection. If your hardware and your ISP have no detectable issues, then you're going to be out of luck as it means the issue lies somewhere out on random routes along the net between yourselves and Bungie, so it's outside of the fixable range of both.