I thought I'd see a ton of examples, but I've been googling and
so far have only find single key examples. Is this "optimal"?
I would like to introduce Option key + Up Arrow and that would
mean an even bigger boolean conditional?

if ( keystate[SDLK_LCTRL] && keystate[SDLK_UP] ) // Ctrl + Up Arrow
{
blah, blah, blah
}
if ( keystate[SDLK_UP] && !keystate[SDLK_LCTRL]) // Up Arrow only
{
blah, blah, blah
}

Well, yes and no. Performance-wise it won't change anything. Those checks count for zilch compared to the rest of stuff your program does (remember, always profile before you optimize).

On the other hand, if you care about your general coding pratices, there is indeed a lot of room for improvement here. Analyze what is checked in wich order, you are checking twice for the same criteria, and the other criteria is checked for both itself and its inverse.

SDL has key modifier definitions for keys like Shift, Control, and Option. The modifiers have the KMOD prefix instead of SDLK.

SDL programs usually check for each key individually when handling multiple keys. Suppose you want to use the arrow keys to move the player and pressing two arrow keys moves the player diagonally. If the player presses both the left and up arrow keys, the player moves up and left. What you would do is check if the player pressed the up arrow key, check if the player pressed the left arrow key, then move the player based on what keys were pressed.

I typed the code off the type of my head so the C syntax may not be perfect, but you should get the idea of what to do.