You want to use two applications simultaneously on your tablet, for example the sell & pick application and an application for your cash register system? This is no problem for the Apple iPad. For a detailed Instruction visit: https://support.apple.com/en-us/HT207582
You´ll need version 1.2 of the sell & pick application to use multitasking.
On an iPad Pro, iPad (5th generation or newer), iPad Air2 or iPad mini 4 the application can be used together with the cash register software (split view). Otherwise you can use “Slide Over” to let it float above your cash register and move it when needed.