[Butcher]

On a 100W cpu with a 0.05C/W water block on it and ambient of 25C, you'd get these results:

220W: Tcold = 6.2C Thot = 43.3C dT = 37.1C
2x220W: Tcold = 4.1C Thot = 56.6C dT = 52.5C
1x172W: Tcold = 11.7C Thot = 40.1C dT = 28.4C
2x172W: Tcold = 2.0C Thot = 50.2C dT = 48.2C
1x172W/2x172W: Tcold = 23.7C Tmid = 52.2C Thot = 60.4C dT = 36.7C
1x172W/2x220W: Tcold = 17.1C Tmid = 45.5C Thot = 66.8C dT = 49.7C

These assume dTmax of 68C for all TECs, 15V TECs run at 13V, 24V TECs run at 21V. These numbers are theoretical, in reality they'd be a bit different.
Note none of these setups will actually burn out the TECs but the stacking really doesn't work well, the top two just can't cope with the heat load.
The best dT is from 2x220W TECs, so if your water cooling system can better 0.05C/W (which is damn good), you would be better with dual blocks. Otherwise a single 220 is the best.