Primoz is right , 2.4ghz is already a very congested band. This alone prevent the effective use of 40Mhz.
Consider the number of channels on 2.4ghz band and assume that 40Mhz, channel bonding has been configured . which means the channel span would be from 2.4 to 2.44Ghz and if we add another 40Mhz , which would be outside of the 2.4 Band and cannot be possible to have such combination. So you will get only one non-overlapping channel in 2.4ghz with 40mhz channel width .
All the APs on same channel is generally not a good deployment , it will cause interference and degradation of the wi-fi performance.
That's the reason we do not configure 2.4ghz on 40Mhz , instead of giving good throughput it might degrade your wireless performance .
I would suggest keep your 2.4ghz band with 20Mhz channel width .