Get access to first bank. You can follow instructions in Quickstart Guide.
On a success event when an end user authorizes themselves for one ASPSP, you present them an option to select another bank.
When building a link, you have to create another requisition where you have to use the same
end user ID and
reference ID (so these two sessions can be connected) but replace the
ASPSP ID of the second bank for a user to be authorized to.