It can be done, but as already mentioned, its not really that great of a combo.
That being said, it could potentially knock you the fuck out.
I once did N2O while being under the influrence of Oxazepam and a small dose of alcohol.
I was pretty chilled out before hand but not even close to blacking out or anything like that.
After ingesting 12 grams of N20, I immediately fell into a deep sleep, only to wake up a couple of hours later with minor memory loss.
I found the overwhelming effect kind of wierd at the moment and did some research.
What I found was that N2O actually shows some minor affinity for the GABA-A receptors (at least, when combined with a true GABA-A agonist) so I think this might have been the reason, or at least have contributed, to me passing out cold after ingesting a N2O dose that I would normally be able to handle without any problems.
So you might want to go slow on the siphon untill you know how you react to the combo.
Here are a few links regarding the connection between N2O and the GABA-A receptors.
http://www.ncbi.nlm.nih.gov/pubmed/9477068
http://www.benzobuddies.org/forum/index.php?topic=51481.0
http://journals.lww.com/ejanaesthes...O_and_xenon_potentiate_GABA_gating_at.13.aspx