So far only 5 (+1) ships in Armada (Including sub variants) has a Dual Upgrade Slot.
MC80 AC: Two Defensive Retrofits.
MC80 BC: Two Turbolaser.
MC80 SC: Two Turbolaser.
ISD I: Two Offensive Retrofits.
Interdictor SR: Two Experimental Retrofits.
The Pelta can get a Two Officer upgrade slots with the Phoenix title. (Only one Pelta in a fleet can get this, so in a way it dosn't actually count, as it is not on the card as standard)
Eventually we will see Dual Upgrade slots in the other types of upgrades, the question is:
Which Dual Upgrade slot will we see first?
Which ship from the Canon or EU (knock on wood) would fluff wise, be justified to have a Dual Upgrade slot?