The IAU cites Wood L., 2003 who defines a satellite constellation as "a number of similar satellites, of a similar type and function, designed to be in similar, complementary, orbits for a shared purpose, under shared control" [1]. So, Sentinel-1 is a two-satellite constellation, yes.

[1]:

https://www.iau.org/public/themes/satellite-constellations/,

https://link.springer.com/chapter/10.1007/978-1-4615-0431-3_...