How to combine two war files into single deploy-able war.This can be achieved by using Maven overlay as part of Maven war plugin. Below steps can be used to achieve the same

Lets say project 1 wanted to include in project 2.

Step 1:

Declare packing type as war in project 1 in pom.xml

<modelVersion>4.0.0</modelVersion>
<groupId>com.merge</groupId>
<artifactId>Test1</artifactId>
<packaging>war</packaging>

Step 2:
Declare project 1 as dependency in project 2 pom.xml

<dependency> 
  <groupId>com.merge</groupId> 
<artifactId>Test1</artifactId> 
<version>0.0.2-SNAPSHOT</version> 
<type>war</type> 
<scope>provided</scope>
</dependency>

And finally add below plugin in  project 2 build configuration in pom.xml

<plugins>
  <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-war-plugin</artifactId>
        <version>3.2.3</version>
        <configuration>
          <overlays>
            <overlay>
              <groupId>com.merge</groupId>
              <artifactId>Test1</artifactId>
            </overlay>
          </overlays>
        </configuration>
      </plugin>

Now final war file in project 2 contains files in both the war files. Please note static content in the first project should be under webapp folder to copy properly in the final war.