Sure, go ahead. The cap is mostly there to keep splashing out to a minimum. If the bigger tank is what you're calling the "expansion tank", it's already vented to atmosphere via the overflow hose. It forms a liquid reservoir for the pressure tank (also called a surge tank, degassing tank and a few other names) to draw coolant back into the engine when things cool off. If it was sealed, little or no water would be drawn back.
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