To put one's palms together — the action of bringing both hands together (e.g., when praying, giving thanks, or making a request).
初詣で神社に行ったら、手を合わせてお願いをした。
When I visited the shrine for the New Year, I put my hands together and prayed.
彼女は食事の前に静かに手を合わせた。
She quietly put her hands together before the meal.
感謝の気持ちを込めて、手を合わせて頭を下げた。
With gratitude, I put my hands together and bowed my head.
Used idiomatically to mean 'to compete against' or 'to face someone in a match' — i.e., to play or contend with an opponent.
明日の試合では、古くからのライバルと手を合わせることになった。
In tomorrow's match I'll be facing a long-time rival.
大会の決勝で彼女と手を合わせたが、惜しくも負けてしまった。
I went up against her in the finals of the tournament, but unfortunately lost.
普段は仲がいいが、カードゲームでは真剣に手を合わせる。
Though we're usually friendly, we compete seriously when playing card games.