What treatments would you recommend in patients with adult-onset Rasmussen’s encephalitis after the patient is on two or more AEDs?

There is no anti-seizure medication of choice. It is a (hemispheric) focal epilepsy. If it is really Rasmussen's, it is a progressive disorder, and eventually, it results in a hemiplegia with progressive hemispheric atrophy on MRI, so surgical treatment (hemispherectomy) should be considered early rather than late. Other treatments like plasmapheresis have been tried with differing efficacy.