If I have say a 6/14 split between wizard and warlock respectively. My wizard technically only has up to 3rd level spell slots. But because the warlock has 3 pact slots up to 5th level, could I copy a 5th level wizard spell and use it in my warlock spell slot?
No. All spells that you can learn/prepare are determined for each class separately. Only the 6 levels of wizard matter to what wizard spells you can learn, copy to your spellbook, or prepare -- up to 3rd level spells. Likewise, the 14 levels of warlock are all that matter when determining your warlock spells -- you get your mystic arcanums and up to 5th level spells.
If I have say a 6/14 split between wizard and warlock respectively. My wizard technically only has up to 3rd level spell slots. But because the warlock has 3 pact slots up to 5th level, could I copy a 5th level wizard spell and use it in my warlock spell slot?
No. All spells that you can learn/prepare are determined for each class separately. Only the 6 levels of wizard matter to what wizard spells you can learn, copy to your spellbook, or prepare -- up to 3rd level spells. Likewise, the 14 levels of warlock are all that matter when determining your warlock spells -- you get your mystic arcanums and up to 5th level spells.