Just been thinking about this a little more.
a = 12
b = 189
n = Floor ( Log10(b) + 1 ) = 3 (number of digits in the second number)
( a * (10^n) ) + b = c
So...
( 12 * (10^3) ) + 189 = 12189
a = 189
b = 1234
n = Floor( Log10(1234) + 1) = 4 (4 digits in b)
(a * (10^n)) + b = 1891234
Not sure off the top of my head how you do Floor and Log10 on the Arduino.
So basically what I'm saying is: Crosh was right (or righter than me anyway). 