So the Imperial Ace TIE Interceptor card allows you to have two modifications instead of one, does this only apply to Modifications proper, ie Stealth Device, Hull Upgrade, Targeting Computer, etc; OR does it mean I can have two upgrades that fall under the same symbol, ie Outmaneuver, Adrenaline Rush, Push the Limit, etc? Thanks

It allows 2 modifications only, so no double Elite Pilot Talents, no double system upgrades etc.

Thanks

However Rebel Aces will have a card that allows the A-Wings to have two EPT's.

The cards in general are "upgrades", while "modifications" is a specific class of upgrade.