Matthew 18:19
Again, truly I tell you that if two of you on earth agree about anything
they ask for, it will be done for them by my Father in heaven.

When Jesus said this in scriptures, He actually meant "any two persons" and
not specifically for individuals in a relationship.
Now, imagine you two in a relation then take this much serious and choose
to pray together... How much more great things happen.

Sometimes, you just need somebody to agree with you in prayer; somebody who
knows how to stand on God's word and intercede with Him for you two. It
yields in two different ways.

Firstly, when you two pray together, you settle joint decisions in prayer

Also, in intercession