"Our children are married to each other" would be one way to say it. It's not a shorter way to say it but it feels more equal or less formal.
Another way might be, "my daughter-in-law's mom". That puts more emphasis on the relationship in a different way. So you're a "mom" more than "mother in law." Similarly, you could introduce her as "my son-in-law's mom."
"My son's wife's mom" could also make it a bit shorter. You could introduce her as "my daughter's husband's mom."
Or to really change it up she could say "I'm her daughter's mother-in-law" and you would say "I'm her son's mother-in-law" so that the introduction is more of an I-statement about who you each are rather than an introduction about who the other is.
Keep in mind that you introduced her to us the same way she introduced you.