What are all the rest of your specs?
SLI allows two graphics cards to work together for more capability.
I do not recommend it.
Dual gpu is prone to stuttering, screen tearing, and increasingly non support in games.
It is hard to imagine a single GTX1080ti being insufficient for anything.
The GTX1080ti will work just fine in your motherboard it is as good as it gets.
You will be very lucky to find one for sale.
The usual GTX1080ti needs both a 6 and an 8 pin pcie aux power lead from the psu.
Some highly overclocked versions require two 8 pin connectors.
Your 350w psu is entirely insufficient.
Power supplies are relatively standard.
Measure the end dimensions.
They should be 150mm x 85mm for a standard ATX psu. The depth can vary, from about 140mm to 190mm.
I might recommend a Seasonic focus 650w unit:
https://www.newegg.com/Product/Product.aspx?item=N82E16817151186
It is easy to replace a psu.
If you have never done it before, take a photo of every place where you unplugged a power connector to remind you,
Parts are keyed to fit only one way. Do not force anything.
If you look closely, the individual pins will have some square and some rounded ends to fit only one way,