〜と相まって
combined with, together with (reinforcing)
Noun + と相まって / Noun A と Noun B が相まって
Sign up free to review this point on a spaced schedule.
と相まって means two factors combine and reinforce each other to produce a result: 天気の良さと相まって、会場は大勢の人でにぎわった, "together with the fine weather, it meant the venue was packed".
The factors add up, for good or bad: talent plus effort, music plus visuals, a weak yen plus a tourism boom, lack of sleep plus fatigue. The second half is the combined effect.
Another shape is AとBが相まって, "A and B together".
It's formal and written, common in reviews, analysis and news. Compare と同時に (N3), "at the same time as", which is only about timing, not combined effect.
Examples
天気の良さと相まって、会場は大勢の人でにぎわった。
Together with the fine weather, it meant the venue was packed.
彼の努力は才能と相まって、大きな成果を生んだ。
His hard work, combined with his talent, produced great results.
美しい音楽が映像と相まって、感動的な作品になった。
Beautiful music together with the visuals made it a moving work.
円安が観光ブームと相まって、外国人客が急増した。
The weak yen, combined with the tourism boom, led to a surge in foreign visitors.
寝不足が疲れと相まって、ひどい頭痛がした。
Lack of sleep on top of fatigue gave me a terrible headache.