no, just because they pulled that off in the House( witnesses who then return to being staff members) doesnt mean it will happen in the senate. A classic conflict of interest. It is why sessions was pressured to recuse himself of overseeing Mueller, remember? its a cornerstone of law since the Magna Carta. Cant be Both , this scenario will play out in the next couple of weeks and we will see, but remember Scotus is involved in this one, it will be held to a very high standard as it will be televised. Cant have the unwashed in the rest of the world thinking the US justice system(as opposed to the house shitshow) allows lawyers with conflicting interest to "try" a case. Another point I would mention are the senators that are running for election, these obviously have a huge conflict and therefore should be excluded as well. I don think its a slam dunk shifty will testify. I will guarantee, though , he will not do both roles.