If I have two classes that are cleanly separated by a single diagonal line as shown below, can I have a depth 1 decision tree using both features on the question x_2 > x_1 or can we not use two features in a single decision query?
I couldn't comment due to the reputation restriction. I think there are some trees built the way you question. Also, for the data you posted, we can do a rotation of data and use the regular CART.
This post explains the detail and related papers: Non-perpendicular hyperplane decision trees