The argument and inquiry families of scientific thinking skills overlap. Yet they rarely are investigated together, with researchers instead focusing on one or the other. Here we hypothesize that inclusion of an additional intervention focused on inquiry will enhance the outcome of an established intervention known to be successful in developing argument skills, compared to the argument intervention alone. An Argument only (A) and an Argument and Inquiry (A&I) group of young adolescents participated in one or the other of these intensive intervention groups for four hours daily over a two-week period. Both groups made progress in argument skills as expected, but the A&I group showed greater gains in use of evidence to support claims, stronger forms of counterargument, and integrative however arguments that connect opposing claims to one another. The A&I group showed a particular advantage in reconciling contrasting claims, an epistemological understanding that lies at the core of scientific thinking. These findings suggest the value of conceptualizing argument skill development in a broader framework that includes an investigative component as well as the skill of coordinating claims and evidence that is fundamental to argument.