@Davo, for the most part I agree with Timinator. Focus on capturing the north and east half (the coastal area) of the map so your AI partner can focus on the western and southern battlefield.
For a bit more detailed of an explanation
This screenshot may be helpful: https://prnt.sc/ok4r1x
The area in the red border was my base (and what I prioritized the first few turns. When AI 2 is about to breach your partner's final defenses (encroach on your border), send him some reinforcements so that you can focus on the seaside battlefront.
Only build cities if you have left over gold, the 5 for +1 isn't worth it in the beginning as AI 2 has a relatively high army count at the start.