I want to do a corded conversion as seen in this link . However, some Makita tools use dual batteries to have a 36V source. That means the voltage from the 18V power supply will be combined to create a 36V source. Will this present any problem
If you're talking about the power supplies like in the link given in the question then you shouldn't combine the two of them to have 36 VDC. Because they will probably not be fully isolated (i.e. they share the safety ground line). Even if they were, it's not practical.
So, instead of picking an extra power supply, you may pick an adjustable/programmable power supply. This will be practical as it can be adequate for both type of devices, but will not be cheap as you might think.