Parallel is not a viable plan unless the the two emitters (diodes) have absolutely identical characteristics (there is virtually no chance of this fortunate accident occurring in the field). Since they are not identical, one emitter till turn on first and cap the voltage below the other emitter's turn-on voltage -- exactly duplicating your results.

The solution is to wire the emitters in series. Don't carry this to far, two or three will usually work, sometimes four. The output of each emitter is lower, but this is typically not an issue.