The attachment required to carry an R-77 on a pylon includes a ram arm that throws the missile downwards on launch and after a fraction of a second delay the motor lights up and the missile flies towards its target.
This twin launcher would need two ram arms with one for each missile to ensure when the weapons are fired they clear the engines and aircraft body before starting their engines.
The R-77 is designed so that its rear grid fins can be folded forward for internal carriage and I suspect that to be able to fit two missiles side by side between the engines of a Flanker they will need to fold the grid fins too.
If that is the case then I suspect that the under engine nacelle weapon hard points could probably have two R-77s as well.
With its new multiple target engagement capacity it makes more sense to carry more missiles now, and develop multiple weapon carrying pylon attachments.