They will be sharing transmission sites - not networks.
They will manage each others sites on an East/West division of the UK and jointly install and manage new 4G facilities but they will not be combining networks.
There is no combination of networks, nor is there any with 3/EE who also share. They have masts that provide access to each others frequency ranges but data, calls and SMS are routed separately to each individual network from the site.
That said, both companies should have similar coverage. Vodafone/O2 sharing outwith 4G data is not expected to be completed before 2016.