This is a poorly worded effect, but according to this tweet from Jeremy Crawford,
Eldritch Cannon does let you spend a spell slot to create the cannon. That slot must be spent specifically on the cannon, not on something else.
So, according to this, the intended use of this ability is that when you make the second cannon, it counts as building a new one, ...
Same pool for the Twins but both can adopt a different one unless concentration is required to do so
Quote from Dragon Compendium p.17:
A pair of dvati twins shares all class abilities and spells
slots between them. For example, a 3rd-level dvati bard can use
bardic music three times per day in total, not three times for each