does one form open the other? Or are they just open at the same time.
You can reference another form by forms("formname")!ControlName.property or Forms![formname]!ControlName.property, but if you are opening the other form you can send info to it using the OpenArgs parameter. In the onOpen event of the form check what is in the OpenArgs (e.g. Me.OpenArgs) https://msdn.microsoft.com/en-us/library/office/ff820845.aspx