In order to set-up a multi-product ideas portal, you need to first set-up a portal for a single product and then add additional ones into it. This seems a bit counter-intuitive from a UX standpoint. It may be better to allow users to specify an ideas portal at the company (or product line) level to facilitate multi-product portals.
Thank you for your idea. Ideas are specific to a single product, but portals are created at the account level. To create a new portal, go to Settings > Account > Configure portals. Here you can create portals regardless of which product is currently selected in the main product selector.