allow sudo to another user without password
sudo can do just that for you :)

It needs a bit of configuration though, but once done you would only do this:

sudo -u user2 -s


And you would be logged in as user2 without entering a password.

Configuration

To configure sudo, you must edit its configuration file via: visudo. Note: this command will open the configuration using the vi text editor, if you are unconfortable with that, you need to set another editor (using export EDITOR=&lt;command&gt;) before executing the following line. Another command line editor sometimes regarded as easier is nano, so you would do export EDITOR=/usr/bin/nano. You usually need super user privilege for visudo:

sudo visudo


This file is structured in different section, the aliases, then defaults and finally at the end you have the rules. This is where you need to add the new line. So you navigate at the end of the file and add this:

user1    ALL=(user2) NOPASSWD: /bin/bash


You can replace also /bin/bash by ALL and then you could launch any command as user2 without a password:  sudo -u user2 &lt;command&gt;.

If you want to be able to switch to any user just use

user1    ALL=(ALL) NOPASSWD: /bin/bash


Update

I have just seen your comment regarding Skype. You could consider adding Skype directly to the sudo's configuration file. I assume you have Skype installed in your Applications folder:

user1    ALL=(user2) NOPASSWD: /Applications/Skype.app/Contents/MacOS/Skype


Then you would call from the terminal:

sudo -u user2 /Applications/Skype.app/Contents/MacOS/Skype