No. The number of fans shouldn't matter, IMO, as long as both the GPUs are of the exact same base model, which is 1080 Ti.
You can SLI those 2 GPUs, but it's not worth the hassle these days. You would be much better off buying a SINGLE powerful GPU instead. Not many games scale well on an SLI/CFX setup. I won't recommend. You need to decide. If GAMING is your only top priority, then, then investing in an SLI setup is not worth it.
If Gaming is not you main goal, then you can consider going for such a setup. Apart from that, your would be using 2 GPUs at a time, so more power consumption and heat as well, regardless of your PSU.
The power draw for an SLI setup is also going to be more. SLI is kind of dead. Implementing SLI/CRFX requires a lot of coding, as well as resources/time. The game developers need to make sure that the game's engine is going to scale well. Taken from Nvidia's FAQ:
Can I mix and match graphics cards that have different GPUs?
No. For example, an XXXGT cannot be paired with a XXXGTX in an SLI configuration.
Can I mix and match graphics cards from different manufacturers?
Using 180 or later graphics drivers, NVIDIA graphics cards from different manufacturers can be used together in an SLI configuration. For example, a GeForce XXXGT from manufacturer ABC can be matched with a GeForce XXXGT from manufacturer XYZ.
Can I mix and match graphics cards if one of them is overclocked by the manufacturer?
Yes. A GeForce XXXX GTX that is overclocked can be mixed with a standard clocked GeForce XXXX GTX.