I suppose asking for different bump maps for the back and front of a one-sided leaf is taking a step too far... ?  ;)

Not at all - just connect them to a top-bottom blender. If you need a different multiplier on each, then premultiply them with a math node. (Don't try to multiply with shades of gray - too finicky and if you decide to change render gamma, it will change meaning.)

If you're using bump maps properly as I've taught, then you already have a math-subtract .5 node involved. Set each one up to do what you need for each map.

Note that multiply on a math subtract, whose job is centering the bump map around the 0 value, can be accomplished in one node - just multiply your input_1 and your input_2 by the same value. For example, if you want the map centered on .5, and multiplied by .16, then on the Math:Subtract set Value_1 = 1 * .16 = .16 and Value_2 = .5 * .16 = .08.

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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)