I believe the U14S optionally supports a second 140mm fan as well. I'm currently running the SP3 version with a second fan. The Noctua website describes the NH-U14S-SP3 as having the following feature:
Anti-vibration pads and fan-clips for second NF-A15 (optional)
For users who want to achieve even better performance by adding a second, optional NF-A15 fan to create a push/pull setup, the NH-U14S includes fan clips and an extra set of custom-designed anti-vibration pads that allow to off-set the rear fan by 5mm to improve acoustics in dual-fan mode.*
That quote about the second fan does not appear on the Noctua page for the DX-4189 version of the U14S. But I suspect you can still add a second fan because the difference is the mechanism to mount the heatsink to the CPU, not the mechanism to attach cooling fans to the heatsink fins.