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.