Some time ago, I released a plugin 'Multiboxes' that improved Zen Cart default multi-boxing calculation when cart content is too heavy to fit in one box. Few people have downloaded this plugin now and a support thread might be needed.
Plugin is here:
https://www.zen-cart.com/downloads.php?do=file&id=2362

It is mainly a modification of shipping class providing new global variables you can use in your own shipping modules. Zen Cart default shipping modules 'perweightunit', 'table' and 'zones' modified to use this capability are included too. Looking at their code should help if you want to do your own shipping modules with multi-boxing feature.

Please have a look at readme file and if you have some trouble ask here.