unify, unite

わせる(あわせる) To unify. To bring different things together and make them fit. Like a band tuning up is called 音合わせ (making the notes fit).   $$$
 
わせる(あわせる) To make two people meet each other. Not used.
 
併わせる(あわせる) A formal word for unite. You don’t need this, either.   NP