I have a T7600 with 3x Quadro M6000 and I can use all 3 GPUs simultaneously without issue. At one point I did try to install a P40 but I couldn't get even a single one working. That said, since you are able to get 1 of them working, I see no reason that 2 can't work.
Do you have 2 CPUs installed? 2 of the PCIe x16 slots will only work with a second CPU installed.