Basically, a Y-harness connects two servos in parallel; hence, both move in the same way commanded by the same channel of the receiver.

For non-computerized radios, a servo reverser can be used, which is a special Y-harness that makes both servos move in opposite directions:

http://www3.towerhobbies.com/cgi-bin...?&I=LXAFN1&P=7

The other option is to install the servos in such an arrangement that suit your needs for each case.

You may find better responses to some of your questions in this thread:

http://www.rcuniverse.com/forum/m_87...tm.htm#8775961

Regards!