I do not believe there is function to concat two clobs. I only see them converting to string and add. However, note that depending upon the size of the clobs this can cause OOM. When you access clos as is they use streaming. Another option is you can write UDF and write that function in the Java to meet your needs.
Clob concat is covered by https://issues.jboss.org/browse/TEIID-1248, but has not yet been implemented - which will be done in a memory safe way, just as with SQL/XML functionality. I'll post a workaround to your issue on the original topic.