The trick is to not use a detectable iTTL flash in the FlexTT5's hot shoe. This means either using tape over the TTL connectors or to simply use a non-iTTL flash or an iTTL flash in manual mode in the hot shoe. This then permits the FlexTT5 to send a "fire now!" signal to the flash in the hot shoe and to the flash connected via the P2 port, both at the same time and in sync.
The proof is in the pudding...
This is where I physically disabled TTL by putting gaffer tape on to the TTL contacts of the SC-28 cable which is attached to the FlexTT5:
Here you can see the cable plugged into the P2 port of the FlexTT5. Forgive the slight blur, this was before I had my coffee and shutter speeds were at 1/8th of a second... lol
The SC-28 cable that is attached to the FlexTT5's hot shoe connector leads to an SB-80DX which is set to manual mode (1/128th). Now at this moment, I did not know that the tape made no difference as long as the flash was in a manual mode, but I wanted to remove that aspect physically for the tests.
The P2 port sync cable leads to a non-iTTL hot shoe connector and in this test, the SB-800 was the flash used here. It also was set to manual mode and a 1/128th power.
Basically, all together, this is how the setup worked. I hit the test button on the FlexTT5 and both flashes fired in all tests (be sure you have disabled stand-by on both flashes!)
That, however, was not the definitive test... I wanted to see if a MniTT1 could trigger this setup... and indeed, it did perfectly as demonstrated below.
I then removed the tape from the iTTL connector and switched around the flashes, where the SB-800 was in the hot shoe of the FlexTT5 and the SB-80DX was connected via the P2 port. With the SB-800 in TTL mode, only the P2 port fired, but when I placed the SB-800 into manual mode, both flashes again fired in sync.
So now that we know what can be done, what cannot be done? Well, as far as the Nikon ControlTL units go, we cannot fire flashes attached to the hot shoe and P2 port of the Flex if the flash that is in the hot shoe is detected as an iTTL capable flash. When this happens, the hot shoe "fire now!" signal is disabled and only the P2 port attached flash fires.
Here is a small hypothetical situation that this could be used to your advantage... lets say that you have used all your ControlTL units and now need to fire off a couple more studio heads but there is nothing left in your bag except some very basic 3rd party triggers and receivers.
Well, one could attach a 3rd party transmitter to that Flex and a couple of 3rd party receivers to the studio heads and your on camera MiniTT1 triggers the whole kibosh without hassle!
This is a nice little tool to add to the chest of tools that the ControlTL units can do. :)