Check the assmbler. The compiler SHOULD generate the MUL A, B instruction to do this, from this code.

But you don't really need it. All you actually want is to concatenate the two registers. You could do:

unsigned char a[2];
int * ip;

res[0] = TH0;
res[1] = TL0;

ip = (int *)a;

That might be more efficient. But need to check if the bytes are the right way around - that could be compiler dependent?