I'm not familiar with the D3200 but assume that it has no CLS/Commander mode. You could either deflect the light as suggested above, provided the slave flash can see it, or turn the flash compensation power down so that its brightness is insufficient to cause much/any impact to the photo and the slave detect it. Alternatively you could use a hotshoe adapter/extension and run a wire to the flash (which is a trip hazard and a PITA).
If you're going to be doing a lot of OFC, then you may as well just bite the bullet and invest in a set of wireless triggers. Whether you decide to go with manual or TTL triggers, or Pocketwizard 344/433Mhz (to use sekonic meter triggering) or 2.4ghz triggers is up to you and will dependant upon your budget and existing equipment. If it's posed stuff, I personally just use manual triggers on a PW frequency so I can meter the flashes with the Sekonic L-758DR. You have time to adjust and fine tune your exposure. If you're shooting moving subjects or if you are on the move yourself with OFC (like at a wedding or event photography), then TTL is usually the weapon of choice.