I've attended weddings before where both of the mothers participated in a unity candle ceremony, where they each lit a candle at the altar, and then both of their candles were used to light a third candle together. You could also ask them each to do a reading, or give a blessing/prayer (if you're religious) during the ceremony.
There is a rose ceremony where the officiant takes a moment to recognize the special relationship of the moms and their bride/groom kids. The bride gives a rose to her mom and the groom gives a rose to his mom. I’ve always liked that one.
Daughter and SIL did this rose ceremony, but each gave their rose to the other mom. At the time of the wedding SIL had been part of our family for more than 8 yrs, starting in their sophomore year of high school, so it meant a great deal to me that he gave me his rose. Also, SIL escorted his mom down the aisle at the beginning of the processional, then I followed escorted by one of the two best men, followed by the wedding party. Their officiant had been SIL's youth pastor and did a wonderful job of really getting know them as a couple and he wove brief sayings from each of us four parents into the service. It felt very personal and family-focused.
I'm half Filipino, so we're adding the Wedding Cord tradition into our ceremony! (this tradition isn't limited to just our culture, so you could do it too!)
This tradition requires two sponsors who will place the cord around you and your partner's heads, to lay it on your shoulders. After you exchange vows, the same sponsors (or different ones) remove them and the ceremony is done! We will probably have his mom and my mom be the sponsors. It's a nice way to symbolize your unity, and makes the Moms feel included more!