Derek Hough & Mark Ballas -- TWO'S COMPANY ... We Bought a House Together!

This is a story about two young people with a special connection who just bought a house together to begin a new chapter of their lives ... Derek Hough and Mark Ballas.

TMZ has learned ... the "Dancing with the Stars" pros -- who also happen to be BFFAEs -- just went halfsies on a quaint little place in the San Fernando Valley.

Official documents show ... the best buds each threw in on the $1.275 mil home ... which comes equipped with a massive closet, fabulous kitchen and a spa bath that could easily fit two.

The master bathroom even contains his and his sinks.