Can tow WordPress sites share a database

Hello Team,

I wanted to know if I have two wordpress sites on the same host can they share a database? Say I have the main website and then have another copy of wp installed in a folder

I wanted my users to be able to use the same user/pass for each site. I know there are support plugins, but I am talking about anything.

I have a site that we use that has a very nice support theme and another for graphic design that has a graphic approval system, and then we have our main site, I know this is the hard way but it has to be like this for now till we build one main site with everything we need it it.

Any help would be great, thanks.