If you want to combine two list fields, you can use ListCombine():
If your Artist fields are separated by a / character, then MC by default doesn't consider each item as a list item - it is just one big string. So you can recombine the list in various ways, replacing the separator characters as desired:
replace(listcombine([artist],[guest artist]), ;, / //)
ListCombine(listcombine([artist],,//,;),[guest artist], ;, / /// )
ListCombine(replace([artist], / //, ;), [guest artist], ;, / /// )
There are plenty of ways - the key is to consider what the combined fields look like, and then replace characters where you need to.