I just updated J2Store module a bit in the file:
/templates/ja_mixstore/html/mod_j2products/Bootstrap3/default.php
Line 28:
<div class="col-xs-6 col-sm-6 col-md-<?php echo round((12 / $total_cols));?>">
and this custom CSS:
@media (max-width: 480px) {
#Section175 .j2store-products .j2store-module-product-row > div.col-xs-6 {
min-height: 425px;
}
}
the products show in 2 cols on mobile now.