Welcome!
A couple of points worth noting:
1. To take a board feed, you may need an attenuator. Board feeds tend to be very, very hot. Some decks, like the Edirol R-44, have attenuation built-in, but I do not believe the DR2D does. Therefore you will likely need an in-line attenuator, or to ask the sound person to dial the signal way down. You should listen on headphones to confirm that it's working.
2. Almost everyone here is going to tell you that the internal mics on any consumer portable are not good enough for recording music. In your case, if you're running a board feed, that will help, but still, everyone will say "Your results will be better using even inexpensive external mics." That is true, and also, it makes it easier to place the unit. For instance you could leave the unit back behind the SBD and just tape the mics to something. Small external mics are quite easy to mount. A good set of small mics and a power supply can be had for under $200.
Also, although I realize that the DR2D advertises that you can do this, it strikes me as tough to get a good mix going on the fly, since it mixes down to a stereo file while recording. Again, I recommend trying this out using headphones, which is tough since the band you will be recording will likely have you in it!
But to get back to your question, I'd put the tascam back by the board. If you can, get a little stand or something that can elevate enough above the crowd to avoid chatter. Depends on the venue what your options will be.