PPG is pretty finicky and the signal more often than not requires cleanup by software algorithms. Double readings are not uncommon and appear even in medical grade sensors. Most sensors used nowadays have multiple emittors or a single emittor with multiple sensors. The "evaluation sensor" with one led and one sensor used for this device will indeed require very precise and stable placement. Even so i'd recommend trying to get the raw signal out of the sensor if possible at 24-40hz (24 and 40hz are commonly used for PPG processing algorithms and usually a good place to start from). working with and visualising the raw signals will give you a much better insight of what you're measuring.
Current supplyed by the Core2 is not gona be a limiting factor in this. Noise of the voltage convertor could cause issues, but this can only be checked if the raw signal is analysed. Often sensors make use of motion sensors to discard readings during detected motion.