The question was asked, how did Tolkien describe the attack on the Trees in Lost Tales? And, not surprisingly, it's a bit more detailed (though parts of the story are completely changed, like the murder of Fëanor's father and the theft of the silmarils occurring at the festival well before this, and this version having no thieves' quarrel.)
Deeming that now is the time to strike while Valinor is yet in uproar nor waiting for Aulë and Tulkas to block the passage in the hills, Melko and Wirilómë crept into Valinor and lay hidden in a valley of the foothills until Silpion was in bloom; but all the while was Gloomweaver spinning her most lightless webs and ill-enchanted shades. These she lets float down so that in place of the fair silver light of Silpion all about the western plain of Valinor there creeps now a dim uncertain darkness and faint lights waver in it. Then does she throw a black cloak of invisibility about Melko and herself, and they steal across the plain, and the Gods are in wonder and the Elves in Kôr are afraid; nonetheless they do not as yet suspect the hand of Melko in this, thinking rather it is some work of Ossë's, who at times with his storms caused great mists and darkness to be wafted off the Shadowy Seas, encroaching even the bright airs of Valinor, though in this he met the anger both of Ulmo and of Manwë. Then Manwë sent forth a sweet westerly breath wherewith he was accustomed at such times to blow all sea-humours back eastward over the waters, but such gentle breathing availed nothing against the woven night heavy and clinging that Wirilómë had spread far abroad. Thus was it that unmarked Melko and the Spider of Night reached the roots of Laurelin, and Melko summoning all his godlike might thrust a sword into its beauteous stock, and the fiery radiance that spouted forth assuredly had consumed him even as it did his sword, had not Gloomweaver cast herself down and lapped it thirstily, plying even her lips to the wound in the tree's bark and sucking away its life and strength.
By accursed fortune this deed was not straightway marked, for it was the time of Laurelin's accustomed deepest repose; and now behold, never more would it wake to glory, scattering beauty and joy upon the faces of the Gods. Because of that great draught of light, suddenly pride surged in Gwerlum's heart, and she heeded not Melko's warnings, but sate herself now night to the roots of Silpion and spouted forth evil fumes of night that flowed like rivers of blackness even to the gates of Valmar. Now Melko takes the weapon that remains to him, a knife, and will injure the bole of Silpion as much as time will allow; but a Gnome called Daurin (Tórin) wandering from Sirnúmen in great boding of ill sees him and makes for him, crying aloud. So great was the onrush of that impetuous Gnome that ere Melko is aware, he has hewn at Wirilómë wherein the likeness of a spider she sprawls upon the ground. Now the slender blade that Daurin wielded came from the forge of Aulë and was steeped in miruvor, or never had he done harm to that secret [?being], but now he cleaves one of her great legs, and his blade is stained with her black gore, a poison to all [?things] whose life is light. Then Wirilómë writhing throws a thread about him and he may not get free, and Melko ruthless stabs him. Then wresting that bright slender blade from his dying grasp he thrusts it deep into Silpion's trunk, and poison of Gwerlum black upon it dried the very sap and essence of the tree and its light died suddenly to a dismal glow lost in impenetrable dusk.
The names are different, of course, but shouldn't be too confusing. Melko = Melkor, Wirilómë=Gwerlum=Gloomweaver=Spider of Night=Ungoliant, Gnome=Noldo, Kôr=Tirion, Silpion=Telperion, and Daurin is meant to be a relative of Fëanor (and was actually Fëanor himself as first written).
So to answer the question, the attack is on Laurelin first, then Telperion, if we follow this example. But, this version has the attack occurring at the lowest light of Laurelin, while the Silmarillion has it occur during the mingling of the Lights. So, if we keep it during the mingling of the lights, we might want to consider attacking Telperion first, as that would be more...subtle. I obviously don't think we should have any trace of Daurin, but if Melkor's spear is Noldo-made, that is reference enough to the idea of Melkor using their own weapons against him. The spear could in some way be damaged by the sap of Laurlin, which would explain why he brains Finwë rather than stabbing him.
