Circuit network question / Kovarex enrichment

I can't put my finger on the solution of this problem because I haven't done such calculations in a long time. Here's what I want to do. Let's say I have 2 centrifuges and I kickstart the kovarex enrichment. I extract both the 235 and the 238 in separate chests. What I want to do now is, once the chest with the 235 reaches >40 I want to just extract the excess in a separate chest. But I want to fully dump 40 from the chest in the next centrifuge. Seems I can't solve this problem with basic wires, I can tell the extractor to only grab >40 but can't set the one for the next centrifuge on <40 because the chest doesn't get instant filled from the prior centrifuge so it will dump everything in the next centrifuge instead of waiting for 41, then extract the excess in the chest and the 40 in the next centrifuge. How do I solve this?