Didn't you have the deads day as a light squat day before? I thought that is what you were doing. There's going to come a point where you can't expect to do them both heavy on the same day at that sort of volume. You also did them after MP's which impact the entire body as well.
You know, Starr, whom so many people think their workouts are modelled after never prescribed deadlifts because he thought they weren't worth the injury potential. Course he was big on the OL type lifts and many people think they have a greater injury potential, but that's a topic for another discussion. I don't agree for a second that deadlfits are so damn dangerous but you gotta at least treat them with a proper amount of reverence.
But it's funny when people like Starr and others that are so reverred, respected, and successfull would completely dump a lift and then you see people doing routines that have in essense people like Starr as the foundation for the idea but they are not only doing heavy deads they are doing them after heavy squats at the same volume on the same day.
Maybe for a rank novice it wouldn't matter too much but you are not a rank novice. It's getting to the point where you better start showing deads some respect and give them their due if you don't want them to bite you in the ass. The fact is the world is not going to end if someone decides to prioritize deads for a day.
As far as you squats suffering...they will suffer more from having technical failure on deads due to fatigue. At the same time it is not like you are going to be able to continue loading the bar for squats thrice a week for very much longer if you are doing that at all and having one day that "suffers" or is even intentionally ligher is not going to hurt your squat progression in the long run. Only the short run.
Honestly if it were me I wouldn't ever have people doing deads and squats heavy/similar volume on the same day. Not if I weren't right there to watch them. And that's another point. You have a training partner, right? Part of the purpose of a training partner is to watch your back. You guys should pay attention to each other and be able to tell the other when it is time to back off. I don't know of course, but your partner may have been able to see a breakdown in technique before you could and stepped in.
I really think there is a big difference from what a qualified coach should prescribe when he can be right there watching and reviewing and what he should prescribe "for the masses". This is the very reason why many of them hesitate, and rightly so, to right programs for the masses. Because, as Ross, said, everybody is different.
So to some up all my speechifying
...You need to respect the deads.