You ask a good question that many people wonder about. The Pennsylvania cremation burial laws allow each cemetery to set its own guidelines about the burial of ashes. I'm assuming
that your parents are buried in a full-size grave that a casket was buried in, and not a cremation grave. Most cemeteries will allow at least one and sometimes two sets of ashes to be buried on top of someone's grave. There is a charge by the cemetery for the digging and filling in of the grave. This is usually substantially less than for the burial of a casket.
Some cemeteries do have regulations regarding the type of urn that can be buried or they may require a vault for the urn to be placed in. This requirement is usually a rigid container that ensures that if a decision is made in the future to move them, it can be done easily. This container or impervious urn will keep your ashes from going into the earth so if that's your wish, be sure to ask the cemetery if a vault is required.
Another thing that may interest you, is that many cemeteries will allow the burial of ashes anywhere on the graves. That would make it possible for you to be buried in the middle so you would be on both parents' graves. Whatever you decide, it's important that you put it in writing and let the person know who will be responsible upon your death. You're wise to think ahead about this so you can have the peace of mind of knowing your wishes will be honored.