I saw in a grow forum that they would use the same fixtures for MH and HPS and install a switch to bypass the igniter when using the MH lamps.
Point being that they used two different color lights.
Maybe the OP is using half of the lights 12 per day, full lights the other 12. Maybe the guys on the grow forum were using MH as the daytime mode:huh:
I don't think the chances of a MH and HPS having the same voltage/current characteristics is all that great, they would need to switch to a different ballast or maybe a different tap of a ballast designed for both - if such a thing exists. Pulse start MH uses an ignitor similar to the ones used with HPS, but again not necessarily same voltage and current.