Let's say I have a Double Coston and one other monster on my side of the field. Can I tribute Double Coston and the other monster to summon Destiny Hero - Plasma using Double Coston's effect (to treat it as two tributes) even though the summoning for Destiny Hero - Plasma is considered a Special Summon.

From my understanding, no you can't use Double Coston. On the Double Coston card it states "used for a tribute summon". Destiny Hero - Plasma is a special summon. Just because you are tributing monsters that doesn't mean it always works as a tribute summon. Monsters are tributed for Ritual monsters and those are labeled as special summons. A normal tribute (one not based around a special summoning effect) for a monster is counted as a normal summon.
However Sheep Tokens can be used to summon D.H. Plasma because they're treated as a special summon, not a tribute summon. Another way to summon is to use a card that summons tribute fodder onto your side of the field. For example Phantom Skyblaster can summon "skyblaster Tokens" onto your field.