confused about combining PAR vs Spectrum

[lildraken]

Would someone be so kind as to clarify how combining PAR's work? I understand that PAR is the measure of light intensity but for instance:

if a bulb emits a PAR of 300, would two of the same bulbs change that PAR reading or does the reading remain the same?

if you combine a bulb with a PAR of 300 with an actinic bulb reading of 200, would the first bulb just wash out the second and the overall reading be 300? or would the PAR be somewhere greater than 300?

finally, people with multiple T5 bulbs struggle to find the right combination of bulbs to maximize looks and growth, but can someone tell me which is more important for growth? Is a higher spectrum better for growth? 10,000 K is inferior to 18,000 K for growth or does it matter?

Thanks in advance.