Attributes released by two IdPs

[Stefano Zanmarchi]

Hi,
I'd like to know if it's somehow possible for a SP to retrieve a set
of attributes from
two IdPs, just one performing the auth but each releasing a subset of
attributes.
And, if the scenario is possible, would one IdP contact the other on
the back channel,
or rather redirect the browser to its front channel?

Thank you very much for any link, information on the subject.
Stefano