[math-fun] A simple naive question

Hello Math-Fun, I would like to transmit two _composite_ numbers A and B in the form of single number C. How can I do that? For instance, if Alice concatenates A et B, Bob will have a problem in reconstructing those numbers out of C: A = 865445 B = 12377007 C = 86544512377007 Is there a clever, economic way to say where C must be split? Or do you know another technique (dealing perhaps with prime numbers)? This has nothing to do with cryptography (I guess) -- just a (silly) question. Best, É.