Each individual chew protects for 12 weeks. Package contains 3 units.

You don't really need Bravecto with NG Spectra as NGSpectra also covers ticks.

Yes you can use them together.

Bravecto protects your dog from ticks including black-legged tick (deer tick - Ixodes scapularis), American dog tick, and brown dog tick for 12 weeks. Bravecto also kills lone star ticks for 8 weeks. According to the manufacturer, the most common adverse reactions recorded in clinical trials were vomiting, decreased appetite, diarrhea, lethargy, polydipsia, and flatulence.