Am I correct that the only difference between these two cards is that the T1/E1 card supports only voice and the G703 card supports voice and data? Or is there more to it?
Can the G703 two port version support voice on one port and data on the other port at the same time?
No, that is not correct.
The only difference between those cards is that VWIC2-1MFT-1T1/E1 suppport voice and data, but VWIC2-1MFT-G703 also supports unstructured G.703 E1 for data.
Basically that is E1 without G.704 structures.
In case you don't need to get unstructured G.703 E1 for data (I really doubt you do), then you are safe with VWIC2-1MFT-T1/E1 which is cheaper.