zeb99:
Actually, it would be firstnumber * 16 + secondnumber.Although this may be more reliable:
result = (firstnumber & 0x0f) << 4 | (secondnumber & 0x0f)
Therefore:
secondnumber = result & 0x0f
firstnumber = (result & 0xf0) >> 4
Now that is a complete answer. C can be so nice and concise if you let it be.
Mark meant well, just having a bad hair day. 
Lefty