My first question is, why do you want to double them up?
At my LDS, the boosters cut off at 3,300 psig, eventhough they are capable of boosting up to 6,000 psig. This particular boost setup has a "safety pilot valve" (not for sure if this is the technical term) which limits the boosting pressure. So if they screw up on the amount of oxygen or helium in the mixture, they can not add anymore since the ending pressure is higher than 3,300. But that's not say the booster at your LDS is like that, they could have a simple booster setup.
You are correct LEDs are the future, but for now they are only capable of meeting 10-15 watt HID, if you want a brighter light, then HID is the way to go. Halcyon, and Light Monkey make a 35 watt and 50 watt HID; it will be decades before a single LED diode will be on par with these high powered HID.
They each have their pros and cons.
LED:
Pro: compact, light weight, more energy efficent, and durable
Cons: non-focusing beam (or at least I have not seen any), some have a wide beam (multi diodes), more expensive for single diode head vs. multi diode head.
HID:
Pros: brighter, focusing beam, preferred by cave and wreck divers
Cons: larger, bulbs are fragile, expensive for the 35 and 50 watt units
I know the pros and cons list can go on and on, but selecting a light is up to you. You will be satisfied with either, LED or HID light. Light Monkey, Halcyon, and Dive Rite make very good units; there are other brands out there but I can not comment on them.
