No, you should not use 40MHz for 2.4GHz. While it is true that 40MHz provides more bandwidth and can therefore theoretically provide higher speeds, in practice it often causes more problems than it solves.
The main issue with using 40MHz on 2.4GHz is that it reduces the number of available channels. In most parts of the world, there are only three non-overlapping channels available for use on 2.4GHz (1, 6, and 11), so using 40MHz will effectively reduce that to just one channel. This can cause congestion and interference issues, particularly in densely populated areas.
Another issue is that not all devices support 40MHz on 2.4GHz. Many older devices only support 20MHz or even 10MHz, so they will be unable to connect to a network using 40MHz channels. Even some newer devices don't support 40MHz on 2.4GHz, so it's important to check before selecting this option.
In general, unless you have a specific reason for needing the extra bandwidth that 40MHz provides, it's best to stick with 20MHz or lower on 2