Created SiteSpecTests and slightly modified SiteSpec.

api/src/main/groovy/com/jessebrault/ssg/SiteSpec.groovy

@@ -15,8 +15,8 @@ final class SiteSpec {
 
     static SiteSpec concat(SiteSpec s0, SiteSpec s1) {
         new SiteSpec(
-                s1.name.blank ? s0.name : s1.name,
-                s1.baseUrl.blank ? s0.baseUrl : s1.baseUrl
+                s0.name.blank ? s1.name : s0.name,
+                s0.baseUrl.blank ? s1.baseUrl : s0.baseUrl
         )
     }
 

api/src/test/groovy/com/jessebrault/ssg/SiteSpecTests.groovy

@@ -0,0 +1,41 @@
+package com.jessebrault.ssg
+
+import org.junit.jupiter.api.Test
+
+import static org.junit.jupiter.api.Assertions.assertEquals
+
+final class SiteSpecTests {
+
+    @Test
+    void ifName0BlankThenName1() {
+        def ss0 = new SiteSpec('', '')
+        def ss1 = new SiteSpec('test', '')
+        def sum = ss0 + ss1
+        assertEquals('test', sum.name)
+    }
+
+    @Test
+    void ifName0NotBlankThenName0() {
+        def ss0 = new SiteSpec('ss0', '')
+        def ss1 = new SiteSpec('ss1', '')
+        def sum = ss0 + ss1
+        assertEquals('ss0', sum.name)
+    }
+
+    @Test
+    void ifBaseUrl0BlankThenBaseUrl1() {
+        def ss0 = new SiteSpec('', '')
+        def ss1 = new SiteSpec('', 'test')
+        def sum = ss0 + ss1
+        assertEquals('test', sum.baseUrl)
+    }
+
+    @Test
+    void ifBaseUrl0NotBlankThenBaseUrl0() {
+        def ss0 = new SiteSpec('', 'ss0')
+        def ss1 = new SiteSpec('', 'ss1')
+        def sum = ss0 + ss1
+        assertEquals('ss0', sum.baseUrl)
+    }
+
+}