Why is there always something missing ? I ask myself this all the time.
You don't say how you are slaving the units. I assume you are using MIDIclock.
It has been reported that the TF MIDIclock implementation is less accurate than it could be. This has been much improved in the recently released PitchFactor, and if sucessful, will appear in the next (no public date yet) TF update. This will send MIDIclock as well.
Note that there will always be a risk of long term drift, due to slight numerical errors – you may set your tempo to 120 bpm, but in fact it may be sent as 120.1 bpm, which will cause a drift of 3 seconds per hour.