Sharing WIFI connection security

Hi guys,

I'm a bit of a newbie to WIFI setups but have a bit of a unique situation coming up in a few months.

I'm renovating my house so it changes from one large house to two individual units. We will be renting out the unit on the other side of the dividing wall.

Stage one of the renovation means both units will be sharing power, gas and water connections (not ideal, I know, but splitting them is part two of the renovation).

So the house will have 1 Spark connection, and both units will be using the WIFI from this connection.

My questions are:

What hardware do I need to give both units their own username and password? Or is this a pretty standard feature these days? I don't want us to be able to access their files and printers, and vice versa

Which hardware is a good choice to be able to give a strong WIFI connection through the dividing wall, and across two stories of a reasonably large house? Or do I need to consider something like Google Wifi and place a few units around the house in both units?

Cheers,

Matt