I believe this is the way it works....
MH (Metal Halide) is best for veg and is in the 5700K range... VS. HPS (High Pressure Sodium) which is best for flowering and is in the 2700K range
With this particular setup that i just polled about.. and bought... there is no interchanging of MH & HPS bulbs... however there are many types of bulbs out there
I think I can answer your qustion... I believe there is a HPS & MH bulb made into 1 light bulb
check this out
http://www.htgsupply.com/viewproduct.asp?productID=53233
also check this "CONVERSION" bulb out
http://www.htgsupply.com/products.asp?categoryID=4&subcategoryID=83&typeID=44
I think you are right... you can use a MH bulb in a HPS ballast and vs versa... but you must get the correct bulbs to do so