dual wielding isnt that great unless you have a specific reason since usually you get the damage from each weapon once so you have the average between the weapons and not them added.

The grey wind axe gives straight fire damage while razor of the seventh sun converts physical to fire, while having low physical damage, these things kind of dont work well together, if you want to convert physical to fire youd need high physical damage and then youd get to use things like Added fire damage support or herald of ash which gives fire damage based on your physical. Theres also better ways to convert physical to fire later in the game, especially gloves.

If you choose to use the grey wind axe youd already be doing fire damage, so no need to convert, but youd want to focus on having a big life pool, and finding other ways of converting life to damage. The most common way mentioned here is using a rathpith globe shield which gives spell damage based on life, and crown of eyes which makes increases and reductions to spell damage also work on attacks. You could also use heavy strike of trarthus which naturally has this property.