The odds of encountering/hatching/taking a shiny pokemon in 3D games is 1 in 4096. Tandemaus and Dunsparce have 1 in 100 chance of evolving into "special" form. By multiplying, the chance of both happening at the same time is 1/409600.

It's worth noting that a Dunsparce transferred from Generation 3 or 4 has a 50% chance of evolving into a Three-Segment Dudunsparce if it has the Hardy nature and the Serene Grace ability. Otherwise, it will always evolve into the Two-Segment Dudunsparce

Thanks!! Sadly I don't have access to Pokemon bank ;-;