Dan gave useful advice while I was typing. Good point about synching with light from a small on-camera flash; I should know, having three bare-bulb light-triggered units.
1. The accessory shoe of the RetinaII is a "cold" shoe: as you can see on the photo that you supplied, there is no electrical contact.
2. Checking my Retina IIa, above and to the left of the lens (looking at the camera front side) there is a small green lever with two positions: M and X. Use X for electronic flash.
3. Below and to the left of the lens (looking at the camera front side) there is a coaxial contact, pointing downwards, that seems to match the (standard) coaxial cable of my vivitar283.
If I had to do what you plan, I would wait for an occasion to buy on the auction site an Y junction and two long cables for the two flashes. Rather than embark into wireless, seems not consistent with the vintage of the Retina. But that's your choice.
Check with a simple one-flash setup that the sync contact of the Retina is functional, before purchasing fancy equipment.
You do not
have to use continuous lighting to take the picture. But so-called pilot lights, located near the flash tube of each unit, help to judge the lighting balance.
There are no Polaroid backs for the Retina