Can I use 1 Spell slot from Warlock to use both Divine Smite and Eldritch Smite in the same attack? Or I must use 2 spells slots?
They need each their spell slot
The two features have similar requirements:
you can expend a warlock spell slot to deal an extra 1d8 force damage to the target
(Eldritch Smite; XGtE p. 56)
you can expend one spell slot to deal radiant damage to the target
(Divine Smite; PHB p. 85)
however, these are separate activations (as they are from separate features) and require each their spell slot. If you want to activate both, you would need to expend two slots (one of which will have to be a warlock slot).