Two step up converters to build dual supply with a virtual ground
While this is certainly a viable design, it might turn out more frustrating than other approaches:

The "virtual" ground at +15 V is actually the output voltage of your "lower" step-up converter instead of the usually very well-grounded actual ground. That means that unlike "true" dual supplies, your approach means that the stability and accuracy of your power supply defines the 0V point – which isn't free of effect on your measurement. You can of course stabilize that by overdimensioning that supply, making sure that there's no oscillation of energy between the two supplies, large decoupling networks... 

But in the end, you want to not go through extra lengths in an attempt to make a circuit less complicated or error prone. 

Really, let's have a look at what we're talking about. This is the CMR curve from the AD620 datasheet:



So we're talking about the difference between 90 dB CMR and let's say 130 dB CMR, or rather 80 dB and 100 dB for a 1 kHz signal. 80dB+ are definitively impressive rejections already (assume you have a common-mode voltage of 1 V, how much of that do you get on the output? How far is that above Johnson-Nyquist thermal noise voltage over your next stage's input resistance?), and I'd be very surprised if adding a second supply to your problem wouldn't introduce more noise than you gain rejection …

Really, for least-noise, high rejection, not using your 12V SMPS to use two further SMPSes to generate a dual supply might be a pretty good idea, even cost-wise.

However, you say there's only one noisy 12V supply – so try to add as little additional noise as possible. 

I'd personally go for something relatively simple, maybe a flyback supply where the secondary side has a center tap to provide your virtual ground, and use relatively massive capacitors to flatten the output voltage, followed by positive and negative linear voltage regulators (negative ones are a bit harder to get, but do exist).

Or, really, generate e.g. +10 V from your +12 V using a positive regulator, and a well-regulated +5 V – and really check if your system really benefits from the additional CMR in a real-world environment.