Tab Mix Plus

I had two windows, one with multiple tabs with incomplete forms, and a second one on a shopping site. Instead of merging the shopping window to the other one with lots of tabs, it closed the multi-tab window, I lost my partially completed forms as it reloaded all, and added them to the single tab window.

I can't find a setting or explanation for this behavior, but I would really prefer that if I have a many-tab window and a few-tab window that it merge and reload the few tabs into the many-tabs.