Question:
Can you use more than the master BM in the field is it possible to have secondary BM?

Answer:
Yes you can have many secondary benchmarks. They will appear like the Master benchmark.

To make them more visible go to Existing Topography Properties:
Right click 'Existing Topography' under Project Explorer > select Properties.
Then adjust the 'Marker Size' and 'Label Size' to make them bigger.