What should this be doing exactly?

join-pattern.js

var Rx = require('rx');

var a = new Rx.Subject();
var b = new Rx.Subject();
var c = new Rx.Subject();

var resA = Rx.Observable
  .when(
    a.and(b)
    .then(function (a, b) { return a + b; })
  );

resA.subscribe(function () {
  console.log(arguments);
});

a.onNext('0');
a.onNext('1');
a.onNext('2');
a.onNext('3');
a.onNext('4');
a.onNext('5');
a.onNext('6');
a.onNext('7');
a.onNext('8');
a.onNext('9');

b.onNext('0');
b.onNext('1');

What you are seeing is correct behavior. This isn't a once only operation. It's just a when either happens in any order, then produce this result. See this test for further details