Are those lenses normal primes or macro lenses? If normal primes, reverse mount the 50mm on the 100mm to get 2:1 mag. But I would buy a 2nd hand lens with a manual aperture ring as the reversed lens, then have it fully open. You can get a 28mm Carl Zeiss F1.8 for a fiver, maybe tenner, off eBay and a reversing ring plus step-up adapter for anther tenner. You will need some light (eg flash, bank of LEDs) so you can stop down the main lens to get a big enough depth of field and a fast shutter speed (eg 1/200). You might want to invest in a focus rail to low you to focus sack. Experiment with a cheapo Chinese £20 one from eBay before investing in something more expensive.