Au pair share - any tips

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We are considering an sharing an au pair with a friend, the au pair will live with the other family but come to us for 2 days a week which we will pay the host family for. Does anyone have any experiences to share? We have had live out nannies in the past but not this arrangement so are unsure of the pitfalls,

Many of our au pairs have worked for neighbours and helped out with extra baby sitting etc. The other family should pay the au pair themselves though - I would think an au pair would feel quite put upon if you were being paid money for her to go and work elsewhere..

Are you recruiting the au pair together - so they know the score before they turn up? I've known people's au pairs do cleaning or babysitting for other families in their ' off duty' hours (and in that case the au pair is paid directly) but not shared like a nanny-share.
Trouble is, if you only pay au pair for her time then the other family is paying for her food, power bills, travel etc for 7 days a week and only getting the benefit of 3 days cover instead of the usual 5. And when au pair works for you she is not 'live in' so therefore subject to the national minimum wage etc.
Would the other family host an au pair anyway, even if he/she isn't shared or are they seeing this as a way to afford it?