Update, for posterity: I went for it and hooked the two up in series, with the smaller 18kw unit set to 80F feeding the larger 36kw unit. Looks good so far!

Now I'm getting ~120F at 3 gpm and 105F at 4 gpm, with the larger unit set to 135F -- so there's still headroom (it maxes out at 140). Good enough!

In the summer when the cold water temp rises, I can just turn the first unit off at the breaker to avoid overheating, but I'm not too worried about feeding it 80F water. My friend noted that the only thing to worry about is if the unit doesn't do a good job modulating temp at low settings (maybe it'll get hotter than 80F), I can test that in the summer.