This plugin has been integrated into Term Management Tools.
While frantically merging tags on a site using this plugin, I realised that it would be easier if you could use the checkboxes, instead of typing each tag by hand.
Now it’s possible. In the bulk action dropdown you will find the Merge option. When you select it, a new field pops up, where you can write the destination tag:
The old interface is still there, for cases where you have tags that are very wide apart.
Also, the plugin is now ready for translations.
This plugin is meant to fill the biggest hole in the tag manament page: the ability to combine two or more tags.
It takes the simples approach that could possibly work: two text fields and a submit button:
It works with custom taxonomies too.