We [1] can eat things with Nitrogen, so I expect a competing form to be completely eaten if they are less effecient.
I don't know enough about Boron chemistry, but if there were enough of them we will eat them too, unles they eat us first.
The first stages of living things are probably very ineficient. If you need a year to make a viable copy, a previus life form will probably eat you before that. I think that two independent origins of life in tha same planet are impossible.
[1] If you include bacteria and archea in "we". And even we (humans) can eat some compound with nitrogen, in particular proteins that mix carbon and nitrogen.
Boron nitride is weirdly analogous (indeed, isoelectronic) to carbon in that there's a graphite-like form (hexagonal BN) and a diamond-like form (cubic BN).
One thing that would hold back BN life is that by themselves B and N form more stable compounds than carbon does. Nitrogen in particular forms molecular nitrogen, which is annoyingly tightly bound.
(My guess is that we still need a few thousand years to answer these questions. We still don't understand too may details about Carbon based life. I'm not very optimistic.)