2 Functions at a time?

Yes, but, most of the time this will be not the best solution to a problem.

As a simple example, two Script objects will run at the same time. Each Script object is given its own "thread", or, as Lua calls it, coroutine (routine being a piece of a program and co indicating that multiple are working together, as in coworker)

Coroutines are not true threads. Only one coroutine can execute at a time; ROBLOX just quickly switches between them. Many coroutines at once move very slowly, which is just one reason to avoid using them too much.

Here is the PIL description of coroutines:

A coroutine is similar to a thread (in the sense of multithreading): a line of execution, with its own stack, its own local variables, and its own instruction pointer; but sharing global variables and mostly anything else with other coroutines.

Lua uses the coroutine library to create new coroutines. Coroutines Wiki Article

routine = coroutine.create( func )
coroutine.resume( routine )

coroutine.create makes a new coroutine, given a function.

coroutine.resume "unpauses" (resumes "playback") of a coroutine object / starts a coroutine.

There are other controls like coroutine.yield that let coroutines produce results.

If you're repeatedly resumeing the same function, you can instead use coroutine.wrap.

playRoutine = coroutine.wrap( func )
playRoutine()

coroutine.wrap returns a function which will resume your coroutine each time it's called (which will usually just be once)

ROBLOX provides two new functions to do this in a little less code.

spawn(func) will create and run a coroutine with a given function. Spawn

delay(func, time) will, after time seconds, create and run a coroutine with the given function. Delay

Here are some examples using spawn:

function blues()
	while true do
		game.Workspace.A.BrickColor = BrickColor.new("Bright blue");
		wait(1);
		game.Workspace.A.BrickColor = BrickColor.new("Black");
		wait(1);
	end
end

function reds()
	while true do
		game.Workspace.B.BrickColor = BrickColor.new("Bright red");
		wait(1);
		game.Workspace.B.BrickColor = BrickColor.new("Black");
		wait(1);
	end
end

spawn(blues)
spawn(reds)

-- OR
spawn(blues)
reds()

I mentioned at the beginning that usually coroutines aren't the best solution to a problem. For example, with the above example, it makes much more sense to combine the two into one function:

function redsAndBlues()
	while true do
		game.Workspace.A.BrickColor = BrickColor.new("Bright blue");
		game.Workspace.B.BrickColor = BrickColor.new("Bright red");
		wait(1);
		game.Workspace.A.BrickColor = BrickColor.new("Black");
		game.Workspace.B.BrickColor = BrickColor.new("Black");
		wait(1);
	end
end

redsAndBlues();

See Also: https://scriptinghelpers.org/questions/11836/can-one-script-run-many-items for more complicated comparison of with coroutine and without