In Workfront a 'Portfolio' equates to a 'Business Unit' in our company and a 'Program' equates to an 'Initiative'.
We find ourselves creating several programs (initiatives) with the same name one in each portfolio because programs are a 1 to 1 mapping to a portfolio (business unit). We have many projects that relate to the same program but they are being sponsored by different portfolios.
Topics help categorize Community content and increase your ability to discover relevant content.
I just was doing some checking and it looks like you can't directly do this, but through the use of custom data you might be able to essentially "tag" different programs and portfolios to connect them.
I also hear that more research will be done about this very type of thing soon, so keep an eye out for when those open up.
Also, I did take a look in the Innovation Lab and it looks like there are at least these 2 ideas that sound similar to what you want to do that you might want to vote for and help move them up in priority.