Not perfect, but a start

I was looking to light a lantern we have internally with a small quantaray remote slave flash. Not the results I wanted but I'll show it anyway..lol.. C&C, and help welcome. As you can see the flash is visible..lol.. and I lite the stones below it with a 430ex

I'd try some variant on wrapping the flash in a cloth. For example, you could buy a cheap white foam sheet from an art supplier (or Walmart), cut it to size, and wrap it around the inside of the lantern. That would hide your flash, diffuse the light, and also cut down its output. If you set the flash to minimum power, you ought to be able to control the output of the second flash to balance the two lights however you'd like.
(And if you bought a colored foam sheet, you could also play with color.)

I like the idea, for me though, the shadows on the rocks below the lamp look a little odd as the rock should be lit from above (the lamp) not below. Perhaps put your second flash on a stand.

Peter

I think I'd put a candle in it, set up the tripod and do a long enough exposure to get the highlights I wanted.

I wanted to be able to control the light