What is the correct Sandhi-Vicched ( संधि-विच्छेद) of 'कवीश्वर'?

The word 'कवीश्वर' is formed via Dirgha Sandhi (दीर्घ संधि):
$$\text{कवि}+\text{ईश्वर}=\text{कवीश्वर}$$

Rule: When short 'इ' combines with long 'ई', they transform into long 'ई' ($\text{इ}+\text{ई}=\text{ई}$). Here, the base word is 'कवि' (poet) and 'ईश्वर' (god/lord).