As you can see, the TU-56 has two independent tape drive mechanisms. The device selector is a small thumb-wheel switch, located just above the read/write head.

The unit with just one tape drive mechanism is the TU-55.
As you can see, the device selector is just somewhat bigger :-)

• TC11 DECtape system manual     (DEC-11-HTCB-D)
• TU56 DECtape transport engineering drawings

The TC11/TU56 subsystem has the following diagnostics:

 
The (x) represents the revision level.

Loading or toggling in this data enables the PDP to boot from the TU56 tape.
This bootstrap is taken from the RT-11 V5.5 installation manual.

    How to load a bootstrap program and start the execution.

The start address is 001000.