So for some reason, when I code some simple movement for arrow keys and shooting with if blocks, when I shoot and move up and right diagonally, It doesn't shoot or move diagonally.

Movement eg:

if <key [left arrow] pressed?> then
change x by (-10)
end

if <key [up arrow] pressed?> then
change y by (10)
end

if <key [right arrow] pressed?> then
change x by (10)
end

if <key [down arrow] pressed?> then
change y by (-10)
end

Shooting eg:

if <key [a] pressed?> then
create clone of [ myself]
end

when I start as a clone
repeat until <touching [ edge] ?>
move (10) steps
end
delete this clone

Reported to Help with Scripts. This place is not where you put topics like these.

I'll move this over to the Help with Scripts section for you - that's a better fit for this kind of topic

If I correspond left to x -10, right to x +10, up to y +10 and down to y -10, the sprite moves pretty smoothly diagonally.

Making your projectiles clones of the main sprite will mean that they'll face the same direction the main sprite is, and head that way. You don't currently have the main sprite changing facing at all though.

The code is for this: https://scratch.mit.edu/projects/722655363/

I used the code for a 2 player game and it works for player 1, but not player 2

Nevermind, I fixed the glitch

So, the glitch was that when I moved diagonally and I was shooting with the n key, the code would stop moving diagonally or stop shooting.
I changed the n key to the j key.