[D&D 5E] A two-player game.

Don't know what type of game your are planning on, but if their characters are more RP then battle play I would suggest giving them options to recruit NPC mercs or something. Then they could control them like a familiar so you wouldn't have to hassle over it. While I like your idea to reduce damage rolls, there are some classes that just don't excel in a straight up fight, like the bard or a good portion of rouges. Now of course this is assuming your players are RP centric. If they prefer the hack and slash method then it sounds like you got that figured out.

I personally don't enjoy the idea of giving the players characters the ability to be "jack-of-all-trades" unless they designed to be so, ie the bard. I personally would encourage RP by working with them so they know some people that could possibly help them with some of their weaker areas. Plus, if there are NPC's that get embedded with the party for a while, there is a perfect vessel to hook them with a plot line.

Ooh! That sounds awesome. I did a one shot with only two people in a 5e game once. It was a blast. There is a whole different dynamic when there is only two people at the game. They almost naturally create a bond together, making for some amazing RP (even between people who have never RPd before).
What I used to help me learn more about a 2-player game was tribality's awesome article on party size: tribality.com/2015/05/19/and-how-many-will-be-in-your-party/
I think it is one of the most useful articles I've ever read when it comes to RPGs.

As for your changes, they're great. When I played with my players, I don't think they were exactly needed. However, battles do start going fast when there is only two people. Less monsters, less people. So I can certainly see where you would need to change some things (for example: you lowered the damage rolls for both the characters and the monsters). It's not like I've played a lot of 2-player games.

I'm going to second this advice wholeheartedly.  When I was younger we often struggled to get a full group, and would only have 2 or 3 (often 2) players.  Back then there wasn't a CR, and we didn't even think about it.  When 3.0 came out with their CR concept I balked at it because it was so rigidly designed for 4, and this is part of the reason. Since then I've come to use it more often and it's a solid place to start, but don't worry about it too much. I think if you just err on the cautious side with your first few encounters, you'll quickly get a feel for what is an acceptable encounter for the 2 of them.  With a little practice it's amazing how good you can get at determining appropriate encounters for a party.  

I'm also a believer that parties will rise (or fall) to the occasion to meet the challenge within a small range. Obviously, you can quickly overdo or underdo it, but I've been involved in groups that had easy fights for a bit that began to seem tough - and when they finally were involved in a real tough fight they panic'ed and thought everyone was going to die....only to win the encounters because they dug a little further into their bags of tricks.  

If you were to make any change at all, I'd say make a couple little expendable tricks available to them.  This doesn't change the long term balance of the game, but gives them some short term options if you misjudge things.  A potion of firebreath, or even a couple healing potions can make a big difference.  Perhaps a wand that has limited charges and can't be recharged.  Things like that they'll treasure and horde until it's absolutely necessary to use it - and when they do have to use they'll remember that tough fight they barely survived.