These are both tough ones to track, so it's no surprise you're having trouble.
Onc. serpens doesn't actually exist. There's no Oncydium species named "serpens." However, within the Oncydium alliance, which includes related genera, there is an Otoglossum serpens (ie, Otglss. serpens.) That's the only related genus with a species named "serpens" registered with the American Orchid Society (AOS). While there's little on this particular species, the culture of the genus looks to be consistent and you can read more here: http://www.orchidspecies.com/indexor.htm.
Cyrtochilum (Cyr) appears to have been once considered part of the genus, Oncidium. It seems to have now been placed by some in its own genus, Cyrtochilum. The AOS lists carderi as an Onc. species, yet I find no specific culture info for it. One vendor listed carderi as a Cyr. species, but with no additional info. Jay's encyclopedia gives Cyr. cultivation info, but doesn't list the species: http://www.orchidspecies.com/indexco.htm.
I hope this helps! Good luck!