As was posted before you could possibly go with an additional B3 but if you are required to go twin how about an AS355NP? It is the new twin star with VEMD and upgraded ngines and such (not to be confused with the 355F models which I have heard not much good about). I did a quick look on ECs website and the useful load on the NP is 2447 Lbs compared to 2253 for the B3. It does burn a touch more fuel at 400 lbs/hr over the B3s 347 and it is a touch slower at 122kts vs 127kts at a higher gross weight. I'm not sure about cost but I would bet it is a good bit cheaper than the bigger twins you mentioned. Many of your airframe spares would be the same and transition training for the engineers would be pretty easy. You just wouldn't be able to put 2 patients onboard unless they were siamese twins!
Just a thought
Max