I don't think I would ever want to be responsible or deal with the stress of handling other clients on a basis like this.
There is too much to go wrong.
When I used to work retail, our company had an after hours call company that was put in place to provide support for corporate clients after hours and weekends. They were supposedly trained and had proper contact info as to who to contact, but they often would simply transfer that call to our location, and we would be stuck trying to interpret what they wanted, or worse still, the average customer who does know or care about call forward or agents, says you have to help them because you are part of X company. It was a nightmare.
I guess my point is, you both have two seperate ways of supporting and dealing with your clients. What happens when something critical happens? What if the customer blames you for something? Will your insurance or his cover it?
My experience in retail has taught me that idiots rule the world and there is a short line between chaos and the world we know and hate/love. So, I always expect the worst, plan for the worst because I've been in situations that I swore I'd never get pigeon holed into again because someone thought it would be easy or hey, this is a great idea.
It's not that I don't think folks should do vacation, but I think we have enough stuff to worry about never mind suddenly dealing with someone else's clients. Unless you are intimately familiar with that business and how it's run, you could be in for a surprise.