Can two detectors for gravitational waves be combined into a very long baseline interferometer? While detectors for gravitational waves are interferometers themselves, the question here is about an increase in resolution resulting from two or more detectors far away from each other.
They do use two or more detectors to triangulate the origin of a source. Most recently the Advanced Virgo detector in Italy was used in conjunction with the two LIGO detectors in the USA. This has been published in Physical Review Letters (Phys. Rev. Lett. 119, 141101 – Published 6 October 2017).

With two detectors the sky area for "pinpointing" the source is quite a large area, but with three detectors the area is reduced by a factor of about 20.