Two very different guitars.
The 594 is a classic Les Paul type with a more vintage sounding Pick ups and different neck shape. Whilst it does offer Coil splitting, these are not very 'single coils' but that doesn't mean they aren't usable at all - just different. You get the option to use both Humbuckers as you would expect from a double humbucker guitar too and overall its a Les Paul with a few more tonal options. Two volumes and two tones do allow for more blending options in the middle position as well.
The 509 is more a modern 'Super Strat' guitar and is a more like 2/3 instruments in one. You can run it like a Strat (SSS) and the Single Coils are more stratty/tele sounding and, like a strat, you also get the middle single coil in between positions (or use it on its own). You don't get the double humbucker option though because the middle position gives you that middle Single Coil. The humbuckers are different tonally too but if you don't like the Pups, you can't change them easily - its a guitar you buy to keep stock (not that that's a bad thing). You mentioned its scale length which at 25.25 is much closer to a Strat too but you also have a trem bridge as well.
The neck carve and scale length do make them feel quite different too but I think they are very different instruments and best to pick the one that will suit your needs and preferences the most - at least when deciding which to buy first. Owning both wouldn't be too much crossover because they are quite different instruments. There isn't anyone that can really tell you what you should buy - its not like deciding between a Gibson R8 or a PRS SC 594 which are very similar - although the PRS does offer better QC and more tonal options so a better buy imo but ultimately, they are similar enough that you may not want/need both. The 509 and 594 are [almost] as different as a Gibson Les Paul and Ibanez JEM...
