Adding a second TiVo box

Hi

I'm due to have Virgin tv, phone & broadband installed on the 25th, in my package I get one TiVo box and one v hd box. I want to upgrade my v hd box as it doesn't have enough features. Does anyone know how much this would cost? Would i have to pay a second installation cost even though the engineer would just be installing two TiVo boxes instead of one TiVo and one v hd?

TIA

Kind regards