I don't know if there's a more elegant way, but this will work. Create an macro in the db you want to call with a macro containing an Openform action for the form you want to open. Then Create a macro (say, mOpenForm) in the calling db containing the action:
RunApp "C:\Program Files\Microsoft Office\Office10\Msaccess.exe" "c:\temp\db.mdb" /x macro1
Here c:\temp\db.mdb is the db containing the form you want to open and macro1 is the macro containing the OpenForm
Now just attach DoCmd.RunMacro "mOpenForm" to a button in the calling form
If you always want the same form opened in the second db, call that macro autoexec and the you can leave out /x macro1
Simon Rouse