I'd recommend that first, you have Samantha identify the members of the other overlord's intelligence networks.
Once she knows where the spies are, she can begin the real fun.
She'll be able to feed both of them false information about each other.
Imaginary plans for assassination, sabotage, corruption of trusted officials.
She'll be able to make them believe that the other is out to get them, without either being aware that she is.
For even more fun, have our devious overlady actually go through with some of those plans, leaving evidence that points towards some seemingly neutral third party. The targeted overlord will think that her rival is merely attempting to throw off the trail.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather