They aren't drives, they are partitions -- and no, without removing HP_TOOLS, you can not have the two partitions combined because the HP_TOOLS partition is right in the middle between them.
I do this ALL THE TIME -- and I keep the OS partitions and DATA partitions separate because I can run weekly image backups of the OS partition and store them temporarily in the data partition, and then move them offline to external storage. That gives me a way to restore the OS in literally 10 minutes, should it fail.
If you merge the two large spaces into a single partition, you can't do that.