I am an Arcane Trickster 3/Wizard 2. Based on the multiclass spell slot table, I have two 2nd level spell slots. These are my questions though:
You cannot copy the spell into your spellbook. You can only copy spells into your spell book that you can prepare (p. 114, PHB, inset box Your Spellbook):
Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.
The multiclassing rules say:
You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.
As a 2nd level wizard, you cannot prepare 2nd level spells. Because you cannot prepare the spell, you cannot copy the spell into your book. Arcane Trickster's spellcasting feature does not use a spellbook, so you also cannot copy it as an Arcane Trickster.
You also cannot scribe scrolls of the spell. The Crafting Magic Item rules (DMG, p. 128) say:
The character must also be a spellcaster with spell slots and must be able to cast any spells that the item can produce.
As you cannot cast the spell with either of your two classes, you cannot craft a magic item that can produce the spell, such as a spell scroll.
Xanathar's Guide to Everything provides updated optional rules for scribing scrolls (p. 133), but it also demands that "the character must have the spell prepared, or it must be among the character's known spells, in order to scribe a scroll of that spell.", so you also could not scribe it using these alternate rules.
