It is possible, but it is not easy to explain.
eg) left block
edit themes/x/theme.html
<{foreach item=block from=$xoops_lblocks}>
<{if $block.title!="(block name for y)"}>
<div class="blockTitle"><{$block.title}></div>
<div class="blockContent"><{$block.content}></div>
<{/if}>
<{/foreach}>
edit themes/y/theme.html
<{foreach item=block from=$xoops_lblocks}>
<{if $block.title!="(block name for x)"}>
<div class="blockTitle"><{$block.title}></div>
<div class="blockContent"><{$block.content}></div>
<{/if}>
<{/foreach}>
If you can't understand with the code, learn smarty itself.