Based on Phaestus's pump comparison article, I thought I'd try to estimate how much flow you are gaining from adding the second pump, and then how many °c you might stand to gain.
The system you are using is probably somewhere in between PH's low restriction setup, and the high restriction setup. Lets do an average and call it a "medium restriction setup".
Putting two MCP600's in series pretty much doubles the max pressure of the pump.
The two rings on the graph show the difference between a one-pump and a two-pump setup. In this case you're going from 5.3 to 6.8 lpm (very ish). That’s 1.4 to 1.8GPM (+30%)
Now, looking at PH's test of the DTek White Water, we see that increasing the flow rate from 1.4 to 1.8GPM only yields 0.5°C. (!)
(not even considering added pump heat, although this probably wouldn’t be very much at all for your system)
Considering the limited resolution of on-board temp sensor readings, maybe this would explain why you see no difference in temperature
