Common for Banshee and Terrorgheist is they are special attacks. They therefore only follow the special rules for that attack.
It is important to understand that out of the box, a special attack that target a single unit can target whichever unit is damn pleases if no further restrictions apply. Now lets understand the restrictions that do apply.
Both attacks require Line of Sight. Had that been the only restriction, then any unit within los could be targeted no matter who was in combat with who. Now the next restriction say it has to be Within 8". So that limit the use quite a bit obviously, but targeting into close combat is allowed since a close combat target can be within Line of sight and within 8" making it a valid target.
Now this reading is clearly confirmed in the FAQ.
Now we move on to the rules were the Banshee and the Terroghiest are different. Keep in mind that it has already been established that any unit within 8" and Line of Sight is a valid target.
Banshee Specific Rule: "If the Tomb Banshee is engaged in combat, her Ghostly Howl can only target an enemy unit in base contact"
Terrorgheist Specific Rule: "If the Terrorgeist is engaged in combat, its Death Sheik may target a unit in base contact"
So the Banshee has an extra limitation that apply while engaged in combat while the Terrorghiest does not. The terrorgheist rule only confirm that yes, you are allowed to do this.
The difference is also made in the FAQ where it is confirmed that the restriction only cover the Banshee:
Q: If a Tomb Banshee wishes to use her Ghostly Howl attack in close combat, does this have to be targeted against an enemy unit that the Tomb Banshee model is in base contact with? (p31)
A: Yes.
So while the Tomb Banshee is restricted, there is no such restriction for the Terrorgeist.