Wanderer wrote:You can only have one valid visa at once, the new cancels the old.
This is not entirely true, the Government recognises that people can take on a Second Job as long as you have a valid visa.
In fact you can work up to 20 hours a week in the same field as the one your COS is valid for without a separate visa.
Source:
https://www.gov.uk/tier-2-general/second-job
9. Taking a second job
You’ll need to make a new Tier 2 (General) visa application if you want to do a second job that isn’t in the same sector as your main job or isn’t voluntary work.
You can do a second job (up to 20 hours a week) if it’s on the shortage list without making a new application.
You’ll need to be sponsored by your second employer and get a new certificate of sponsorship if you make a new application.
You can’t:
apply until you’ve started work with your first sponsor
start work with your second sponsor until your application has been approved