Tabindex on buttons combined with selected textbox

I have a situation where I have two groups of controls on my page.
Group 1:
2 Textboxes and 1 button
Group 2:
2 Textboxes and 1 button
When im writing in group 1, the tabindex of the button in group one should be the first.. When i'm writing in group 2 the tabindex should change to the button in this group.
 How can I do this?

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