English Translation
The two runners reached the finish line at the same time.
I have never heard other native speakers saying 決勝線. At least, no TV sports news reader says 決勝線.
二人の走者は同時にゴールした。
二人の走者は同時にゴールテープを切った。
are much more common.
If it’s in an oral conversation, the original sentence is very likely to be interpreted as:
The two runners arrived simultaneously at the stadium to watch the final game (of baseball, football or something like that).
決勝戦 (けっしょうせん) is a homophone, and we use 決勝戦 quite often.