Is it viable to Start the BFR mars burn while the ship is attached to one or more refueling boosters? (Imagine interplanetary booster) Using their combined engines the ship can achieve greater velocity reducing the travel time to less than 3 months. Also, the tankers will refill the ship and on separation, the ship will have maximum fuel again. Fuel that can be used to decelerate to achieve stable mars orbit. All the refueling boosters will go back to Earth so everything is still reusable. The BFR is connecting now back to back, but imagine the old way of the ITS.
Ignoring the issue that the refueling mode is expected to be tail to tail, and thus blocking the engines, you are trying to re-invent staging.
However, with 6 or 9 (IAC 2016 differed from 2017, and will likely differ from reality when it actually flies) or whatever the current design has in terms of Raptor Vac engines, the booster is quite sufficiently powered. There is probably just enough thrust already for the boost to Mars.
So extra thrust is not helpful. The extra fuel is helpful. Thus unload it and ditch the mass of the refuler booster, why carry extra weight that is not needed, nor helpful?
No, this is not viable for SpaceX. If they used a booster to do the Trans-Mars orbit insertion burn, so the upper stage can save it's fuel, then the booster would not be recoverable. The booster would be traveling too fast to turnaround and land back on Earth. Elon Musk is all about not throwing away his rockets. He would see this as not being viable.