Code:
Authors:=Lee Child and authors:=Steven Gould
you don't need advanced search
To see a LIVE sample of how multiple searches work :
Open the Tag browser (if closed)
Expand Authors: C : Click Lee Childs:
HOLD the control key
Expand (authors) 'G' : Click Steven Gould
you can now let up on the control key
The search box is a working example of a 2 author search