My MIL has all sons so I made a real effort to include her in the planning. I had one dress shopping trip for both mums, although ultimately I bought my dress just with my mum. As details were decided, I would send her photos or links so she knew what we had chosen.
A few other things we did, which might be things you could consider:
We set our guest list and then told both sets of parents that they could invite any extras they wanted (within a certain limit) as long as they paid for the cost per head of each. This meant they each had certain people who were special to them, as well as us.
In terms of a secret project, I also asked my mil if she could find any specific items belonging to my DHs grandparents which she could surprise him with and give him to carry with him on the day. Not only did DH love this, it also gave me and MIL a shared plan which he wasn't aware of.
We ensured that both mums (and surviving grandmothers) were given special corsages to wear on the day.
We arranged for the memorial lamps in the church to be burning in memory for family and friends on both sides of the family and put a note inside the order of service.
We asked both our mums to be our witnesses (explaining that we wanted the names of all four parents on the certificate) and during the signing of the register we arranged for the organist to play my FIL's favourite piece of classical music.
And after the wedding, we arranged to visit the family graves of my MIL parents, allowing them to see my husband lay his buttonhole on the grave, and taking with us some of the leftover centrepieces which we used to fill all the pots. It looked beautiful, was special for my DH and his family knew that we had thought about both sides of the family. (My bouquet wasn't thrown but was also taken to my brothers grave by my nan). As a religious family these things meant a lot.