Be clear what box you have, it matters. TiVo & V6 both run TiVo-software, TV360 runs Horizon software - and are fundamentally different in how their multi-room setups work. https://www.virginmedia.com/care/tv-fault/which-tv-box - also TiVo & V6 have the TiVo-logo in the top-right corner of the menu, TV360 doesn't.
If you have a V6, then an additional box is just that - additional box. Entirely separate from your existing V6 with its own hard drive and storage, but with multiroom streaming options.
If you have a TV360, additional boxes are minis without hard drives. All recording capabilities are on the master box which retains the one central pool of recordings. You don't mix boxes on the same account - you either have all TiVo/V6 or all TV360.
Connectivity is otherwise identical - every VM TV box needs a co-ax for live TV and internet connection (ideally Ethernet to your home-hub) for online services & streaming. If you don't have a co-ax connection point where you want to use the new box, you'll need a manned tech install.