[RightDeve]

For double charge, aka tandem, ATGM, you'll see in their info screen "DC-ATGM", under Notes besides ammo loadout.
Top attack, this is an attack mode, so you can see it on the yellow bar when an ATGM unit capable of doing that, is firing. You can see it clearly if you set Message Delay to high in Preferences.

The game models them generally as it is in real life. The first charge of DC-ATGM would defeat outer parts of the armor, i.e anti-HEAT or ERA, then the second charge, the steel armor.
Top attack, attacks the top part of an AFV, i.e, the shot impacts on the top armor instead of the normal sides/front/back armor.