Referencing $index of a repeat at higher level

I have a basic structure of

repeat1

[code 1 here]
repeat2

[code 2 here]
here I want to do something like this (code shortened)

<input class="form-check-input" type="radio" value=""  dmx-bind:name="input[{{repeat1.$index}}]" >

I want to reference the $Index of repeat1 from inside repeat2 to use it as an array index

I cannot work out how to do that?
anyone got a solution?

Thanks