CBS bought Last.fm in 2007 for $280m (?173m), one year before it acquired MP3.com as part of its $1.8bn (?1.1bn) purchase of the CNet Group.
MP3.com was launched by Michael Robertson and Greg Flores in 1997 as a legal music-sharing service.
In 2001 it faced lawsuits for its My.MP3.com streaming service before being acquired by Vivendi Universal in May that year, thereafter being sold to CNet in 2003.
CBS Interactive Music Group president David Goodman revealed MP3.com was now set for a re-launch towards the end of June.
"It is an interesting brand as it has global recognition and still gets around 3m unique visits on a monthly basis," he said. "It's mainly from search, as when people look for free music online, 'MP3.com' comes up. We believe there is a lot of opportunity to refresh it."
The company has already outlined its plans for the two platforms to US labels and managers and will present them to the UK music business this week.
While both Last.fm and MP3.com will exist and be run as standalone services and brands, they will feed into each other in a more structured and cross-promotional fashion.
"We will be starting off with 1m tracks on MP3.com, available for free legal download," explained Goodman. "It's primarily for new and unsigned acts that are mainly the result of leveraging the Music Manager on Last.fm - which allows people to upload their music to Last.fm Radio. It reflects a lot of things we are trying to do with Last.fm."
For CBS, the move is intended to drive traffic between the two sites. It already does something similar with its Radio.com brand in the US, where all the radio stations operating under Radio.com link back into Last.fm when users click on the player to find out more information about the artist currently playing.
"We have a really diverse set of assets in the music space - from listening to editorial," suggested Goodman. "From an artist perspective, we are able to do so many different things. And from a user perspective, we are able to leverage all these assets to give people more information about artists that they are interested in."
With the rise of streaming services meaning possessing music files - and therefore the MP3 - is arguably less important to consumers than it once was, there are some doubts that CBS can make MP3.com work.
However, Goodman argued that the service was focused on brand awareness rather than file format.
"It's a globally recognised brand," he said. "We don't have to reinvent the wheel here in terms of creating new technology and editorial. We can create another viable global opportunity for the group as a result of that."
Part of the strategic thinking is to use online searches for "MP3" to direct users to the Last.fm platform, Goodman suggested.
"MP3 surfaces a lot in search today," he said. "Even though the site is functionally broken and won't be re-launched until June 21, it's more about leveraging the brand equity and the brand awareness of MP3.com."
While this specific move interlinks two CBS-owned properties, Goodman said that the future for digital music services lay in them working together, rather than trying to operate in enclosed niches.
"Most things tend to coexist," he argued. "Last.fm has grown as iTunes has grown. There is more interest in music than ever before and people use a variety of services. That's the reason we scrobble 600 different services so that we have the most informed data about what people are listening to around the world."
A strategy of building complementary partnerships between a variety of CBS-owned properties, as well as linking them to outside services, will therefore shape the company's thinking for the foreseeable future.
"The idea there is going to be one de facto music service is completely unrealistic and for us it's about being open," Goodman stated. "We are part of a community and we like being part of that community. It also affords us a tremendous amount of opportunity to grow."