My idea of a better way would be:

* If you just need a boolean of success vs failure: use END_STREAM vs RST_STREAM.

* If you additionally need metadata of failure reason: use the existing length prefixes that gRPC has, and additionally add a bit to indicate data vs trailer. Then implement your own trailers inside the HTTP2 stream to indicate success vs failure and failure reason. Sure these trailers won't get HTTP2 compression like native HTTP2 trailers, but that shouldn't be a big problem.

Using 2 websockets would be confusing because things could arrive out of order from how they were sent. And one websocket could fail but not the other, leading to a confusing mixed state. The whole reason for trailers was to make failures less confusing by having error messages in them.

Also, using websockets goes against the whole gRPC design idea. They wanted native HTTP2. We don't need websockets to fix the problem, we just need to implement trailers inside the stream instead of using native HTTP2 trailers. Implementing trailers inside the stream can be done with native HTTP2 streams or with websockets inside HTTP2 streams. It's a smaller change from the current protocol to put the trailers inside the native HTTP2 stream than to add websockets to the mix then implement trailers inside that.