If you have trouble playing one-hand chord notes simultaneously, let the keyboard teach your hand the shape of the chord.
1. Play and hold the chord with keys at bottom.
2. Notice the chord’s three-dimensional shape in the hand. Keeping that hand-shape fixed, lift the hand to shoulder level or above, and move it around, being free in elbow and shoulder.
3. Still maintaining the hand-shape, bring the hand down in a continuous stroke to the bottom of the keys (with no flexing at the wrist), playing the chord in its original location on the keyboard. If the notes all sound at the same time, you’ve succeeded. If a note sounds too late or too soft, then its finger lost either its position in the hand-shape or some of its “stabilizing force.” In the latter case, study Soft Piano Playing, parts 3 to 6.
4. When you play the chord reliably, use it in the piece it came from. Start a few bars before it appears. As as you approach it, establish its hand-shape in your imagination, and—at the last moment—in your hand.
I learned the usefulness of this approach from the opening “Promenade” of “Pictures at an Exhibition,” where both hands have four-note chords on successive 8th-notes, no two chords in a row having the same shape. Playing these with fingers—it became painfully obvious—is “how not to do it.” Play instead with arm-strokes, changing hand-shape on each stroke, as you might change the working end of one of those multiple tools people have in their kitchens, where heads for various functions can be snapped into the same handle. At the piano, your arm is the “handle,” and the tool-heads are your hand-shapes for the successive chords. You play chord-chord-chord; and the instant before each one, your hand snaps inaudibly into the shape for that chord, like a grace note.
Maintaining a chord-shape, especially while playing, involves some (shh!) tension in the hand. If at the same time we are free in shoulder and elbow, then we have “tension distal to relaxation.” This jargon means simply that the tension is farther from the body’s core than the relaxation. This has a distinctive feel that’s familiar to pianists. 
