I think as per the other answers this is feasible in theory, but may not work out well in practice.
I'm reproducing this graph from an article in our blog, Exploring the 3.3V Rail (thanks PM). Note that this graph shouldn't be considered definitive (keep reading):
Simple enough -- the green line is the 5V rail, tested via the breakout. The load is additional, i.e., sunk via a pin, and the Pi itself (actually a B+) is headless and idle. Part of what this illustrates is the 5V rail isn't as well regulated as the 3.3V rail (it isn't regulated at all, in fact, beyond some overcurrent/overvoltage protection on the microUSB jack).
If you look around, I think you will find a Pi 3 needs nominally ~450 mA to operate properly, because it will draw that much when the processor revs up high enough. Based on the graph, if the rail were providing an even 5V, at 450 mA it is going to drop to 4.6V -- not so good. At that point you run the risk of Pi #2 cutting out (or both, since as per Steve's comment the voltages will probably match up).
Why that drop occurs is debatable (see comments below) and maybe somewhat inevitable (due to "some 5V components [being] subject to changes in resistance and conductivity under load"), or may be in large part because a linear supply is used (for which "the voltage provided...will vary with changes in load impedance", which has been frequently observed by users here particularly WRT the greedy Pi 3).
I.e., YMMV -- but it should be easy enough to check your own scenario with a cheap multimeter. Just busy loop (x4) Pi #2 and see what happens.
Is this a good idea or am I going to blow stuff up?
You won't blow anything up -- if you try hard, you could possibly trip the polyfuse on the first Pi. How wise that is or how often you should do it I don't know, but I would stop after round 1 and think about where all the draw is.
However, done sanely that is unlikely, and the worst thing that could happen to the second Pi is it intermittently reboots due to voltage drop. This could cause filesystem corruption, but not physical damage.