warlock pact slots

So my warlock has just gained a level. I'm wondering if it will be rule-safe if I multi-class my Warlock into another Warlock Class? The initial skills, spell list may overlap but it will be worth it if my character will be both a Pact of Chain and Book! Although a Warlock 2 / Warlock 2 may...

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact ...

Because the Warlock's Pact Magic is so unique compared to regular spellcasting, Warlock spells work a bit differently. Spells which might be weak or too expensive for other spellcasters like the Wizard are frequently better options for the Warlock since spells are always cast at the same spell level.