Perhaps the suggestion was made from an European standpoint? Two accounts in Euros within the SEPA area [1] means no costs for currency conversions or bank transfers between your accounts.
Well, the bank in question holds GBPs, converting to/from Euros to GBPs seems incredibly inconvenient and has no real benefits. Most people get their wage in a single currency and use a single currency in their every day lives.
I have a family member who gets their pension in Canadian dollars but lives in the US. Since he lives in the US he has no need for Canadian dollars. He says that his money loses a quarter of its value converting from Canadian dollars to US dollars. I don't know if that's true, its what he told me.
That's because the exchange rate has gone down - not from frictional transaction costs. The worst ripoff forex fees are in the 1% range. You can do much better if you look.
Further it's relatively easy to keep USD in a Canadian bank account if you want to.
Most banks I've ever done business in the EU with will let you keep a currency account, in select currencies. Having two (or more) accounts in different countries is not particularly difficult then, it's just like having a different bank account. There are usually no capital requirements either, although there may be an annual (but small) charge, and you may have to all and actually talk to someone to get it set up since it's usually not self service.
Even so, the vast, vast majority of Europeans have income and expenses in a single currency, in a single country.
If they don't, then within the Eurozone they still don't necessarily need two accounts. It might be convenient, if it enables using local systems to pay utility bills on two properties, for example.
If currencies are being changed, SEPA doesn't help.
[1] https://en.wikipedia.org/wiki/Single_Euro_Payments_Area