That is a serious question linds, so let me try for a serious answer:
1 - Yes, it is cost. With each DMD costing upwards of £10,000 per patient, per year, you can see how fast the cost would mount up.
2 - But, if you took two or more medications, and one worked, how would you know which one?
3 - Then comes the question of interactions. The BNF (the prescriber’s bible) will only list the interaction of drug A with drug B. Until we have people prescribed two DMDs there will be no evidence of any interaction. If they are also on one or more non-DMD medication, it would be anyone’s guess as to what caused the interaction if there was one.
To give you an idea, when I was on Copaxone, I was also on Gabapentin, plus four more for a heart condition. Now, I am not on Copaxone, but the same four heart drugs, plus Gabapentin, plus one for the bladder. and one for muscle spasms. So far, no interactions, but increasing fatigue - tell me which one is responsible and I will think about stopping it. That is the reverse of point 2.
Finally, point 4 - Without a serious trial of two DMDs together, there would not be enough evidence that two DMDs gave a cost effective benefit (and NICE, who rule on these things are primarily concerned with cost/benefits). Then, just to test (trial) each DMD with each other DMD would cost many millions of pounds. No drug company would fund this, just in case their star product was not effective when given with a competitor’s product.
See the problem? The idea might work, but no-one will ever know.