The basis of modern legato organ technique is piano technique. Ever since modern organ technique was first formalized by Jacques Lemmens (more on this history you can read in this Orpha Ochse's book)in the middle of the 19th century, the legato touch became a norm even for baroque compositions for a long time. Lemmens’ method was perfected by a few other organists, like Marcel Dupre and Harold Gleason who published their own organ method books. Traditionally, normal touch for much piano music is also legato. I am not suggesting that baroque music must be played legato on the piano too, but I want to demonstrate how all pervasive romantic piano tradition was to organ paying.
Piano keyboard is often harder than organ. If you compare various organ keyboards to that of the piano, you will find that to depress a key on the organ (without couplers, of course) often is easier than on the piano. This is especially true on electronic organs, on electric action organs, on pneumatic action organs with barker levers, on electro-pneumatic action organs, and even on small tracker action organs, such as positive organs. This feature of the piano keyboard gives you a lot more resistance. In other words, if you play technically challenging fast exercises, like Hanon virtuoso pianist exercises on the organ for some twenty minutes, your fingers are likely to be less tired than if you play them on the piano.
On the piano all inequalities of the touch is much more visible than on the organ. Try to play an excerpt of any organ composition on the piano and you will notice right away how easy is to play one note too loud with your thumb or too soft with your pinky on the piano. In other words, you must work very hard too make all notes sound dynamically equal on the piano. This is so because of the dynamic possibilities that piano action allows. Therefore the stronger and longer fingers often play too loud and shorter fingers too soft. To equalize all notes, by the way, is our goal when practicing organ music on the piano.
Piano practice develops finger dexterity. If you play exercises and organ music on the piano regularly, gradually you will notice how much easier it will become for you to play in a fast tempo. Your touch will become light and swift, your virtuoso passages will sound truly virtuosic. Such technically challenging pieces, as Prelude and fugue on BACH and Fantasy and Fugue on “Ad nos, ad salutarem undam” by Liszt or Sonata by Reubke will pose no difficulty to you.
Piano practice develops finger independence. If you ever have noticed in your organ playing that technically advanced pieces sounds sloppy, practice them on the piano. All these technical challenges arise because our fingers might not be independent enough. You may notice it very clearly if you play a scale in a very fast tempo on the organ. It will sound legato, of course, which is not necessarily a bad thing. What is worse that this scale may be performed unequally.
In other words, if you record this scale and play it back in slow motion, then you may be able to hear that a few notes could be stuck together and others would sound too detached. Therefore, piano practice helps in developing finger independence needed for technically challenging music.
If you practice your organ pieces and exercises on the piano regularly, chances that you will achieve a high level in organ playing are quite good. Piano practice will give you finger dexterity and independence that will propel your organ performance on to the next level. When play piano, you can practice playing pedal part of organ compositions on the floor. That will save you much precious time. However, never forget that organ touch is very different from piano touch and practice accordingly.
If you would like to know more about the practicing techniques on any keyboard instrument, I highly recommend Keyboard Practice Skills by Elaine Grover. This compact book presents a comprehensive presentation of all the basic practice techniques needed for successful mastering of keyboard skills. Chapters include "Warm-up Exercises," "Creating a Practice Routine," "Learning the Basics," "Developing a Practice Mind," "Using Practice Skills," "Building Momentum," and others.
By the way, do you want to learn to play the King of Instruments - the pipe organ? If so, download my FREE video guide: "How to Master Any Organ Composition" in which I will show you my EXACT steps, techniques, and methods that I use to practice, learn and master any piece of organ music.