I learned guitar first: following that keyboard was easy but not very logical.

The difficulties are to do with playing rather than theory - many theory books are keyboard based, and to read and understand those books, a knowledge of piano layout is an advantage: however, there are now many books which are theory for guitarists, and don't rely on keyboard.

Differences and points of confusion: guitar, all closed scales are the same in all keys. Piano - all scales have different fingering. Chord shapes are easy on piano; complex on guitar. Bending notes doesn't work on piano. You don't tune a piano. Alternative tunings are not used on piano. Ornaments are played differently. Notes occur once on a piano, six or so on a guitar.

Sometimes learning a second instrument can be confusing: I find that the Double Bass is confusing since it is tuned in fourths, not fifths (like most bowed strings), but has no frets like the electric bass. Similarly, I find that I use different fingering for Violin and Mandolin.

Musical knowledge is completely transferable - timing, emphasis etc, but instrument knowlwedge/skill not so. IMO

Stu