Does not allow you to twin-link a heavy flamer.

If you check the VDR from Chapter Approved 2004, which is the last version that GW printed, you'll see that Heavy Flamers are not allowed the Twin-Linked upgrade. This is not a bug.

EDIT: if you look at the Weapon Upgrade chart on the same page as the Twin-Linked rule description it clearly shows that under the FLAMER column the TL option is NO. Flamers can only have the Co-Axial and Mega-Weapon upgrades.