Yes, you can double smoke just about anything. Cold smoke it, then hot smoke it as you normally would, using the normal hot smoke method for that item. Today I'm double smoking some bacon, for instance.
Obviously you should only double smoke something if you want a lot of smoke flavor [
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As to time and temps, my last batch of atlantic salmon took an hour and fifteen minutes at 180 F to get to an internal temperature of 140 F. YMMV.
As to what made your salmon oily, maybe your cold smoke oven temp was too high. What was it? If your temp was above around 80 F, and you smoked it for so many hours, that may explain why it came out oily.
