1) in WB A, in the VBA editor, rename the VBAProject to something meaningful, in the Project Explorer Pane.
2) with WB A still open, in WB B, in the VBA editor, select Tools->References, and check off the entry for the VBAProject name of WB A
3)in a module in WB A, you should be able to type the VBAProject name of WB B, then a period, and a list of all of the VB Objects in the VBAProject should be visible... select the name of the userform, then type .Show
Caveat: whenever WB B gets opened, so will WB A... you cannot close WB A until WB B is closed, and WB A will not close automatically when WB B is closed, unless you add code to do so.
It's the only way I know to get it done... unless you copy the form to the other workbook...