functions.php

<?php

/**
 * Two column tablet classes
 *
 */
function be_two_column_tablet( $classes ) {

	// First, we make sure we're in the grid loop.
	if( ! apply_filters( 'is_genesis_grid_loop', false ) )
  		return $classes;

	// Add this to all posts
	$classes[] = 'tablet-one-half';
	
	// Every other post gets this
	global $wp_query;
	if( 0 == $wp_query->current_post % 2 )
		$classes[] = 'tablet-first';
	  
	return $classes;
}
add_filter( 'post_class', 'be_two_column_tablet' );

style.css

@media only screen and (max-width: 960px) {

  .one-third.tablet-one-half,
	.one-fourth.tablet-one-half {
		width: 49.166666666667%;
	}
	
	.one-third.first.tablet-one-half,
	.one-fourth.first.tablet-one-half {
		margin-left: 1.6666666666667%;	
		clear: none;
	}
	
	.one-third.tablet-one-half.tablet-first,
	.one-third.first.tablet-one-half.tablet-first,
	.one-fourth.tablet-one-half.tablet-first,
	.one-fourth.first.tablet-one-half.tablet-first {
		clear: both;
		margin-left: 0;
	}
}

@media only screen and (max-width: 767px) {

	.one-third.tablet-one-half,
	.one-third.first.tablet-one-half,
	.one-fourth.tablet-one-half,
	.one-fourth.first.tablet-one-half,
	.one-third.tablet-one-half.tablet-first,
	.one-third.first.tablet-one-half.tablet-first,
	.one-fourth.tablet-one-half.tablet-first,
	.one-fourth.first.tablet-one-half.tablet-first {
	  margin: 0;
	  width: 100%;
	}
	}

}