There is quite a lot in the archives about Nanny Shares, so worth doing a search - then ask a more specific question, as I think the lack of a specific question may be why you are not getting many responses.
You need to have a contract between you and the other family, which sets out things such as agreeing on when you take holiday, what happens in the event of someone wanting to leave the arrangement, details of who is paying for what, whose home the care will take place, plus many more things.
If the care DOES NOT take place at your home, then it's not really any better than a childminder... as you are still taking your children to the other persons home - not able to leave them in bed, leave them in PJs, leave them eating breakfast etc.
From what I've read on here about nanny shares (as I've never done a nanny share), the nanny will usually be paid more than if they worked for just one family. However that increased amount is then split between the two families - the split may be 50/50, though more likely to be 60/40 I suspect if all care takes place in one home (the person providing the location pays less, as they have all the heat, light, wear & tear costs).
And how much cash am I going to have to fork out (bearing in mind we're paying under a fiver an hour for excellent childminding, but can't afford much more and am hoping to return to work on much reduced hours after this one)?
It is hard to give figures until a lot more detail about the share has been decided. Also it can vary on location and the level of experience the nanny has.
As you are paying under £5 an hour now for 1 child, I would expect that the cost in a nanny share may not be that much less than what the childminder would charge for 2 children. This is due to all the added on costs that come with a nanny, such as Employers NI, activity budget, mileage payments - where as your childminder may be charging an inclusive fee.
How much do you actually pay your childminder now... and what is included in that? For example... your childminder may charge extra for meals, though that cost may be not be more than it would cost a nanny to make the meal at home and could be less cost... as the childminder may be splitting the cost of the meal between more children.
Apart from for cost reasons... why do you want a nanny share?