seeing two people (i.e. romantically) at the same time; two-timing
bifurcation; fork; branch; dichotomy
fence-sitting; playing it both ways; parting of the ways
彼氏が二股をかけていたことがバレた。
Her boyfriend was caught two-timing.
彼は二股をかけてしまい、深く反省している。
He ended up two-timing and is deeply regretting it.
道が二股に分かれていた。
The road forked.