Here is a thread about CYCLES, not SUPERFLY, that confirms that CYCLES is capable of using more than one GPU at a time.

Is this true of SUPERFLY? If so, how does one go about choosing more than one at a time? (I have NO GPU and cannot test, nor did the manual clarify the matter.)

Link to CYCLES thread regarding multiple GPU

Here are the salient points from that thread regarding Cycles:

"The more GPUs, the more tiles being rendered simultaneously, so two GPUs will cut rendering time almost to one half. Other than rendering, only one GPU will be used for calculations and to refresh the screen. The memory accessible for rendering is going to be limited to that of the card with the least memory."

I, too, would like to know, in case I want to buy some cards, whether buying two cards gets me a faster render or not.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)