Ryanair
I guess you mean a little bit like how they now operate from Liverpool and Manchester. A lot of the more regular running major city routes go from Manchester eg. Brussels, Madrid, Milan, Rome. Plus Manchester has a higher frequency on the typical sun routes like Alicante, Malaga, Palma etc. than Liverpool.
Liverpool is now a more random mix with a lower frequency on the Spanish sun routes but more in the way of the traditional smaller Irish, French and Eastern European airports.
I'm not sure IF Ryanair do eventually open a base or at least expand more significantly in Cardiff whether that will cause a similar split between Cardiff and Bristol. I'd say that as Bristol is arguable the "primary" airport of the two that major city routes such as Milan and Barcelona would be more likely Bristol than Cardiff. However Bristol doesn't seem to have all that many city routes with Ryanair. For instance they still use Girona rather than Barcelona El Prat and other cities like Berlin, Rome, Venice etc. they don't operate at all. I'd say EasyJet is probably the reason why.
Unless they add more city type routes from Cardiff, I can't really see a split between certain types of routes/markets that would suit Bristol/Cardiff. I think Cardiff could end up replicating something along the lines of Bournemouth's network of almost exclusively summer sun routes to Spain and the Canaries, with little effect on Bristol.