Des and Tinny decide to conclusively determine if it's nature or nurture when it comes to their criminal genetic origins. Des and Tinny further decide that the only test that will work is a heist with their fathers followed-up by an assessment of their skills and abilities as crooks.
That both of their fathers are in prison does pose a few problems, though these were easily solved by a double jail-break. Of course, just because their fathers are out of jail doesn't mean they are necessarily up for a sentence-lengthening heist.
Through the power of a 'get the gang back together scene' a la Blues Brothers everything is arranged for the big heist.
It so happens that the bank Des and Tinny are casing had just hired Jake and Mal to perform a security assessment after they'd noticed a few flaws.
Just as the heist is about to start the staked-out Doyles notice that something's up. Instead of calling the cops - because that's not their style - they decide to intervene themselves.
Once the Doyles have apprehended the 'gang' and realize who their dealing with they decide to quietly unjailbreak the fathers and just give Des and Tinny a stern talking to. Of course, after this unexpected turn of events Des and Tinny still have no idea if they have a genetic predisposition to crime or not? B'ys, they're some rotted at Jake and Mal for ruining their tidy experiment.