Not entirely accurate - you can be resident in more than one country. It does mean you are liable for tax in more than one country, but it's quite easy to be resident in more than one country.

If you fly to the UK for more than 45 days a year (partial days count as a full day = so flying Friday night and returning Sunday night counts as 3 days), you MAY be classified as a UK tax resident (dependent on some other factors, like earning money there, having a place to stay, etc). The more days, the more likely to be considered a resident.

Each country has it's own criteria, and they generally don't care what you do with other countries... if you meet their criteria, they charge you taxes and consider you a resident.

EDIT: for UK residency:
https://www.gov.uk/government/upload...nal_078500.pdf

I note that the lower threshold has recently been changed to 16 days, which means it's now really quite easy to fall into this..