Will sharing slow down my Internet connection? (by 2 users)

yes and no, depending on the following, see what happens when browsing is that, when you click on a link, the computer uploads the address of the link then waits for the inflow of the reply which is called the download, when its finished loading the page it stops downloading, meaning the modem is now on standby, so should anyone else in your network click on a link, he will enjoy the full speed of the modem, since the other user is not at that time using the modem, he is probably reading the contents of the page he downloaded earlier. But suppose both of the users download a file maybe a movie from the net, or both wants to video stream a clip say like the youtube AT THE SAME TIME, this means that they will both be downloading and the connection's resourses will be divided among the users, since the modem is serving them both at the same time. if the connections speed is 512kbps it shall be divided in two to be 256kbps or 32KBps, BUT to answer your question directly the slow down of the connection will depend on the nature and the manner of usage of the connection users.

Sometimes it will depend on how fast your net goes, common causes of using additional comps or laptops are freezes and lags. The thing is try not to use so many machinery that require the internet