SERVER-106983 shows an example where we can only push one of the two eligible predicates down into the index scan because the index is multikey. It might be that one of the predicates is more selective than the other, thus resulting in a different plan. However, right now we just pick the first one in the iteration order. We should enumerate both plans so that we can pick the better one. This isn't restricted to or pushdown, however. See this test case - we should also be able to enumerate the plan that has bounds on a.c and d.f.