You will have to pay UI for both up to the max. So, I believe you pay UI on the first $7500 of pay in MD; if you have one nanny, you'll only pay it for her. If you have two, you'll pay it for the second also. UI is usually pretty cheap, though.
Worker's Comp depends on the state. In MD, it was mostly based on annual pay, so the two nannies should come to the same as one working full time. Where I am now, in GA, I pay by the person for domestic employees, so two doubles the worker's comp rate.
All other taxes are income-based, so would be the same.
If your original nanny was getting overtime, I imagine two nannies, neither receiving overtime, would more than pay for the additional UI.