@Nikunj , You can use copy table utility to copy a table into another table. Since you have the same rowkey format and column family, it should be equivalent to a union. It is documented here https://hbase.apache.org/1.2/book.html#copy.table .
For example, you want to copy table t1 into t2, then
bin/hbase org.apache.hadoop.hbase.mapreduce.CopyTable --new.name=t2 t1
If you want to copy tables t1 and t2 into t3 then,
bin/hbase org.apache.hadoop.hbase.mapreduce.CopyTable --new.name=t3 t1
bin/hbase org.apache.hadoop.hbase.mapreduce.CopyTable --new.name=t3 t2
Note that you should create table t3 first before executing above commands.