Is there a way to make this kind of setup

[proud heath]

I want to make an enclosed biobricksetup by turning all the kindlevine extract into limestone. I want to make a setup like in factorio where the priority lane (kindlevine - limestone byproduct) will only allow input from the supplement lane (limestone from miners) if there is space ie prioritizes using the byproduct than the mined limestone.

[dense salmon]

Best way would be probably a second T next to the priority line, with stack inserters moving items where there's space into the main priority line. There is no logic with belts as yet