With no restriction and no interference (what's the difference between "restriction" and "interference"?) you will essentially not have any static pressure.
To the original question, this is little different than two centrifugal pumps in series. Much like that, most people who answer have no clue about which they speak. I will give you the "general answer" without specifics as specifics depend on the actual fans, resistance, etc.
When you add a second fan or pump in series the flow rate will go up. Each fan or pump still follows its characteristic curve, however. Each device is passing more flow, but more flow equates to higher losses. Those higher losses are divided across two devices. You need to find the new point on the characteristic curve where the new losses / 2 meets the curve. You can easily determine absolute bounds for this. With no resistance, flow would truly double (not truly possible). With zero flow, static pressure would truly double (no benefit as without flow there is no cooling). In the "real world", I'd hazard that adding a second identical fan would probably bump net air flow to the tune of ~1/4 to 1/3. This is probably (as shown by Phaetus' data) nearly indiscernable to the CPU.
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