MS SQL Server: Do multiple queries in a batch ever execute in parallel and if so what happens when the second query is dependent on the first?
Batch statements are only ever executed serially in the order they appear in the batch.

Now, if you have two statements sent to the server by two different batches, they will run independently and essentially simultaneously (locking and latching aside).

Take for example the following code:

CREATE TABLE #t
(
    i int
);

INSERT INTO #t (i) VALUES (0);


The CREATE TABLE always runs prior to the INSERT INTO statement.  Consider this:

SELECT 1;

SELECT 2;


The above code will always be ran in order, i.e. SELECT 1 runs first, then after it completes, SELECT 2 runs.

There are multiple ways independent batches can be ran simultaneously, including the use of Multiple Active Result Sets, or MARS, however none of those affect the serial processing of statements within a single batch.