No, it is not recommended connecting the High Level and the Low Level simultaneously. These two inputs are run on the same circuit internally on the REL and will result in far too much gain. Connecting the High Level and Low Level can result in unnecessarily over taxing the REL circuitry.
When possible, connect the High Level Input to your main L-R amplifier channels and set the processor (in a home cinema system) to "Full Range" or "Large" in the Speaker Set-up Menu. Using the High Level Input and connecting to the speaker output terminals is one of the unique secrets of REL’s success and will maximize sub-bass performance.
The primary purpose of including a Low Level input is to allow for problem solving when a Class D amp proves to have a grounding scheme that even REL Pure D™ cannot solve or when using a single REL with a set of Monoblock amps. It is permissible and, indeed, highly desirable to connect High Level and the .1 circuit at the same time for surround sound applications.