This is a tricky area. By trying to create a stronger cipher (and that is
what you are doing, combining A + B to make cipher C) you are putting
yourself above the cryptographers ... who presumably tried to make A and B
quite strong already.
I am surprised no one has observed the combination of ciphers A and B may be no
better, or even worse, than either A or B by themselves. As a trivial example,
double ROT13 is much worse than single ROT13. As a real-world example, double
DES is no better than single DES.
It appears to me, at my present level of ignorance, that an abundance of
caution is appropriate.