Assuming you have some buffering so the comparators swing exactly 5V..
solving numerically to minimize the error squared of the two thresholds and the two hysteresis (using solver software)
R13 = 30.000K (defined)
R14 = 2.923256628K
R16 = 16.07788776K
R21 = 1142.70829K
R22 = 1061.133891
Obviously you could scale those values higher or lower. I happened to pick an exact value for R13, based on arbitrarily making the divider current about 100uA and thus having feedback resistors in the 1M ohm range.
That makes the voltages at pins 3 and 6 2.000 and 1.7000 with both outputs high- with the respective output low they each will switch 50mV lower- 1.9500V and 1.6500V
I simply calculated the current voltages given resistor values (assuming both outputs high), then calculated the two (high and low) resistances looking into the divider from R21 and R22, and from there the hysteresis with a 5V change- 5 * Rthev/(R21 + Rthev), for example.
To roughly estimate the resistor values, you can ignore the feedback (we know it's relatively small voltage change), assume a divider current of (say) 100uA and then you know that:
R13 = (5V - 2V)/0.1 = 30K
R14 = (2V - 1.7V)/0.1 = 3K
R16 = (1.7V)/0.1 = 17K
Just roughly, looking into the node at pin 3 and ignoring R22, we see R13 || (R14 + R16), so the feedback resistor R21 should be roughly 4.95/0.05 = 99 times higher, or about 1.2M. Similarly, looking into the node at pin 6 and ignoring R21, we see (R13 + R14) || R16, so R22 should be around 1.1M.
As you can see, those guesstimates are not far off at all, and it's possible to just fiddle a bit with them in Spice and get close enough that (say) 1% resistor tolerance will dominate.
C14 is a really bad idea- the op-amp will oscillate, also C21 and C22 are not a good idea either. To get the output to snap you should not delay the feedback.
Such considerations simply don't apply to the normal applications of Schmitt triggers. Schmitts are generally used where the input slew rate is much less than the output slew rate. With a slow input, the trigger point is not clearly defined, being affected by other factors such as noise and feedback from the switching transients as the output changes. Once the trigger point is reached, the positive feedback pulls the threshold quickly to the other level, and oscillation is prevented.
However, it is entirely true that Schmitt triggers exhibit metastability, and this was demonstrated in 1977/1979 by a series of remarks in the IEEE Transactions on Computing, by E.G.Wormald and Thomas Chaney. See http://fpga-faq.org/FAQ_Pages/0017_Tell_me_about_metastables.htm for a discussion of the subject.
So basically, Schmitt triggers are not much use for fast signals. For slow signals, they are very effective, but in these cases the low frequencies make metastability much less of a problem, since high sampling rates don't make sense, and there is relatively more time for metastable conditions to resolve themselves.
Best Answer
Easiest is to difference-amplify first, then the output is ground-referenced and the familiar hysteresis circuit just works. Ground, here, is a 'constant reference'.
Like this:
simulate this circuit – Schematic created using CircuitLab
Slightly more tricky, one could use a series resistor and a switchable high impedance current source (LM13700 can fill that function) to put (in the form of current times resistor) a series voltage offset onto the (+) comparator input, polarity determined by the comparator output; that would change the input signal slightly, though.