0.7mm is quite thin, you would be about there with one layer of 200gsm and one layer of 450gsm carbon. That would give you a thickness of around 0.65mm, so you could go a bit stiffer by using a 650g layer instead of the 450g to give you a thickness of roughly 0.85mm. Sort of depends what strength you need from those panels. Not sure what weight bi-axial you are thinking of using, but I suspect your layup would be substantially thicker than the 0.7mm you are aiming for.