If the Doctor had been more capable of thinking instead of just providing raw power, every bit of it he had, not snarling, not growling, just bleeding energy and light into the Master's effort, he would have made some sort of joking comment that would have sent them right back to snarling at each other. Fortunately, he was nowhere near that coherent. He wasn't thinking, he wasn't doing anything except continuing to pour himself and his mind into helping the Master, getting those things out into the open, and then gathering every scrap of self and effort he could manage into that final, determined, and desperate effort.