In one of my individual interviews, a leader of the congregation had not heard anything of companion synods. Each synod in the ELCA has a companion synod in the ELCA and a companion synod somewhere else in the world.
In the United States, the companion synod of the Southwestern Texas Synod is the Southwestern Washington Synod which extends from the Tacoma area south to the Oregon border. Our two synods sit together at church wide assemblies.
Our international companion synod is Iglesia Luterana Costa Rica. We often collect synod assembly offerings for Costa Rica and help them in their ministry to the poor in their country. I visited our church headquarters in Costa Rica on one of my Spanish immersion trips to Costa Rica.