Well as a primer an object can have as many UV mapping channels s you wish by changing the mapping channel in the UV map modifier When you change the channel you are asked if you want to copy the previous map into the channel or abandon the previous map to create a new one.
The normal process for UE4 is to the first map for the colour map and the second channel for the lightmap. The rule is the lightmap can not be tiled or overlap but the editing requirements and process is still the same as the first channel following the requirements.
Best practice once you are done your edit you can bake the channel into the object by collapsing the stack. At this stage the object contains 2 UV mappings, one for the colour map and another for the lightmap,and once imported into UE4 the second channel will be selected as the lightmap channel, which you can change.
If for any reason after the fact you can edit any channel by placing a UV edit on the stack, select the channel you want to edit, an abandon when asked. once done collapse the UV edit to bake the channel.
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