What version of mojoPortal are you using?
Bing search seems to be working fine on this site, though it is not configured as the primary search it is enabled at the bottom of the search page and it works.
Only thing I noticed is that my Bing account key is 44 characters long, but not sure if they are all the same length.
I do know that at some point things changed when they moved Bing to be an Azure service and we updated mojoPortal to work with it but very old versions of mojoPortal probably no longer work because they used the older api that has been shut down since they moved it to Azure services.
I'm not sure from what you are saying whether it is the bing api returning a 403 or if it is the search results page on your site that is doing that.
The one key should be all that is needed. I noticed on this site that I have the key set both in site settings and in user.config
<add key="BingAPIId" value="my44charkey" />
Hope that helps,