This is going to be a little shorter than it could be, but I think the answer can be made pretty simply. I'll try to swing by and add sources later.

Lutherans and Calvinists were very distinct, legally and socially, at the time. German Calvinists had reputations not too dissimilar from their Puritan brothers in England—as extreme, weird, and unattractively stoic. While Lutherans and Calvinists were theologically more similar than Lutherans and Catholics, Lutherans generally thought that Calvinists went way too far in changing the Church itself, while Calvinists though that the Lutherans were still too "Popish". This led to a very sharp division between the two.

Moreover, and perhaps most importantly in the lead-up to the Thirty Years' War, the Peace of Augsburg (1555) only recognized Catholicism and Lutheranism as legal religions—Calvinism had not yet grown to a size large enough to earn recognition. This meant that all Calvinists in the Holy Roman Empire, particularly those who were princes (and there were a significant number by 1618), existed in a social and legal grey area.

This division would prove fairly crucial, as a Calvinist (Frederick V, the "Winter King") effectively began the TYW, and received only tepid support from Lutheran princes. As the War progressed, however, Lutherans and Calvinists were forced to work together because Ferdinand II's successes posed a substantial threat to all Protestants. This solidarity was reluctant, but anti-Catholic sentiments pushed them closer together (though never very).