What it means
と相まって means that one factor combines with another factor to produce a stronger, broader, or compounded effect. It is mainly used in formal or written language, especially when discussing circumstances, qualities, or causes that reinforce one another. The construction normally follows a noun and presents the preceding noun as one of the contributing factors.
Usage note
と相まって is more formal and literary than a neutral expression such as “together with.” It usually describes mutually reinforcing factors rather than a simple list of things that occur together.
How to use it
豊富な資料は、専門家の分析と相まって、研究の方向性を明確にしました。
The abundant materials, combined with expert analysis, clarified the direction of the research.
豊富な資料は、専門家の分析と相まって、研究の方向性を明確にした。
The abundant materials, combined with expert analysis, clarified the direction of the research.