[Viliam1234]

> I usually go with "if one of us is on holiday".

I would expect this to get responses like "you need to take your phone with you on your holidays" or "you should plan your holidays so that they don't happen at the time when the application is most needed".

In my experience, having the bus factor of one was not a problem for the company, it was a problem for me, because I needed to do a lot of negotiation about when to take my holidays. (After a few years I gave them the notice, and then it suddenly became a problem for the company.)