A travelling companion is someone who it is deemed the trip depends on eg. husband and wife - one couldn't travel without the other.
*Each underwriter defines who is a travelling companion differently - check the policy wording*
It is advisable that all travelling companions are on the same insurance policy or have insurance policies with the same underwriter, especially if they have pre-existing medical conditions. This is because if the holiday is cancelled due to illness and it is not due to the person insured with us then they will not be covered. If they are on the same insurance policy then all people involved will be entitled to claim.