Dremel is mostly used for SQL-like queries in logs processing while Tenzing is largely used to run SQL-like queries on BigTable.

That's exactly the question I have. There seems to be couple of hints about it, e.g. in section #4.8:

"Tenzing has read-only support for structured (nested and repeated) data formats such as complex protocol buffer struc- tures. <...> The engine itself can only deal with flat relational data, unlike Dremel [17]"

And from section #5.4 I assume that currently they use Dremel query engine, but are in the works of creating another one.

Dremel aka BigQuery has a dedicated execution engine, roughly an order of magnitude faster than MapReduce for typical SQL queries