The existing list of shareholders is duplicated. Then, each shareholder can decide to sell one while holding the other - and others can decide to buy one without the other. It's possible that some entity may make a bid for one of them in the future. But at the moment, Comcast's owners look to be spread among many institutional shareholders, without anything, and more importantly any person, dominating, so without either converting to a non-profit (unlikely, let's face it), this is probably the best you can hope for.
On edit: I see Wikipedia says that although he only has 1% of the equity stake in Comcast, Brian Roberts has 33% of the voting power. The OP article says
Comcast Chairman and co-CEO Brian Roberts will continue to be actively involved in the leadership of both Comcast and NBCUniversal, working in partnership with the CEOs of both companies, the company said in the statement.
So if that does mean he retains 33% voting power in both, then I guess they will still be significantly linked.