I never knew you can do this but saw it on another blog, not sure if it was posted ever on DDF.
Yo Can Transfer BA Miles To Iberia To Lower Fuel Charges
For example the total cost for a one-way award in business class from Madrid to Chicago is 40,000 Avios points plus $75.62 in taxes.
The same exact award booked through British Airways would cost you 40,000 Avios points plus $262.84.
How to do it: Go to
www.britishairways.com , go to My Account, Click on Combine my Avios, and follow the instructions.
Only thing is in order to Combine both your accounts must be open for at least 90 days.