Trying to Build a voltage-driven smart shunt/threshold switch
So, the crowbar circuit makes a lot of sense; by the way, do the same for Vin &lt; 0, so that you can avoid damage if someone connects the voltage source the wrong way 'round.

The rest doesn't.


V == 7 V (exactly) doesn't exist. Your voltage is a natural, random variable, and you can't even measure anything in this world exactly (course you, Heisenberg!)
Instead of trying to switch over between different two different supplies, simply use one supply that can do both, step up and step down, continuosly.


That supply is more complicated than a Boost or a Buck supply individually, but if you incorporate that you avoid having both, and avoid having to design a clever switchover that avoids transients and damaging the supplies in situations where the voltage oscillates around the nominal output voltage, it's worth it. 

I'm a bit surprised: you designed something based on a TI chip, but TI's product tables actually have a selection guide that let's you select "Buck/boost or SEPIC" as architecture! 

After pluggin' in your 3V &lt; Vin &lt; 35V and Vout=7V I get 5 results. Lo and behold! Your MC33063A is one of them!

So, don't design one boost and one buck with two MC33063A, design one book/boost or SEPIC converter. Sadly, TI doesn't supply a great Application Note telling you how to design your buck/boost converter. Now, that's not a big deal, since the MC3360xx is not a TI invention (but a derivative of one amongst the oldest mainstream SMPS converters in existence), but exists from many suppliers, one of which is bound to have a good description of how to design one, but let's stick to things that come ready-made for our confused minds:

The TI Webench designer (terrible name) allows you to design a 3 V – 35 V in, 7 V out, 1.25 A converter. It recommends an LM3481 in a flyback configuration.

However, going by the LM3481's datasheet, SEPIC would (at least at first sight) work, too.