The trick is recognizing the rhythmic pattern of the combined beats.
Use this phrase "Nice cup of tea". Say it out loud.
This is equivalent to:
3/4 time: Quarter note, eighth, eighth, quarter.
Now RH 3, LH 2. Beat on your thighs:
Together, R, L, R. (keep looping the pattern, to the rhythm of the phrase above)
For RH 2, LH 3:
Togeether, L, R, L
....
Now once you get that there is an excercise to further develop your skill on it.
For the following count OUT LOUD only one pattern: either in 3 or in 2.
R 3, L 2, count in 3
R 2, L 3, count in 3
R 2, L 3, count in 2
R 3, L 2, count in 2
Play the pattern continuously, without stopping rhythm as you change the variation. The pattern loops so when you finish the 4th variation, you go back to the 1st one.
....
For future reference the pattern for 3 against 4:
The rhyme: "Pass the golden butter"
3/4 time: dotted eighth, sixteenth, eighth, eighth, sixteenth, dottend eighth
R 4, L 3:
Together, R, L, R, L, R
R 3, L 4:
Together, L, R, L, R, L
Exercise:
R 4, L 3, count in 3
R 4, L 3, count in 4
R 3, L 4, count in 4
R 3, L 4, count in 3