Is it illegal to commingle a couple's ashes when burying them?

My grandpa and nanna put me in charge of the funeral and disposition arrangements. They want their ashes to be placed in one urn and buried after they pass away. They are so adorable and want to stay together and also have their ashes mixed. I think it is the sweetest expression of their love for each other. I’m just not sure if transferring both of their ashes into a single run is allowed.