My first visa to the UK was sponsored by my cousin and his family, he included his passport page, his wife's passport page, both their payslips, their joint account + his own personal natwest account and tenancy agreement, etc
This would not be classified as joint sponsorship.The main sponsor was your cousin and he basically included his family details in the application.
This is what I have advised teh OP in teh above post.
In there all you need to put is that both you and your wife are working and earning jointly say a sum of £3500 per month, which would be sufficient to pay for the travelling, living and expenses cost of your in laws for the 3-4 months that they are going to be spending in the UK.
I have read the link you have attached below but I cant find anything in there which says joint sponsorship is allowed. It refers to the word "Sponsor" not "Sponsors".
Joint sponsorship is where two different people not closely related sposnsor a visitor to the UK. Thsi would not be allowed as rules require one person to be liable , take the undertaking to be wholly resposnsible for the visitor during his stay in this country.
If joint sponsorship was allowed, who would be responsible and liable?, the more sponsor the bigger the headache for the people in authority as to who to blame if things go wrong.Furthermore the more the sponsors who are willing to pay the visitors cost, hardly any appliaction would be turned down
This is why one person (sponsor) is only allowed.However there is no harm if that sposnors wife who may be working can show joint funds to prove that they have sufficient funds to pay and cover the cost of their visitor.
