Aye, you can only maintain concentration on a single spell at once.
Both Haste and Call Lightning are concentration spells. Thus casting Call Lightning will end the concentration of Haste causing the Lethargic debuff from Haste ending.
In order to use Haste and Call Lighting, you'd need to have someone else cast Haste on your Sorcerer or utilize Potions of Speed (Which will provide a Haste effect for 3 turns without needing concentration)
