Use MAX680 charge pump positive side only?
Based on the idealized schematic on the datasheet,  the two charge pumps are completely independent. Omitting the c2 and c4  caps will make the negative charge pump do nothing, as there is no path to switch.

But using a dual rail charge pump like this is... odd. unless you have some stock you need to use,  you are better off using any number of single rail positive charge pump.