Nigel's right. There's no way you can switch seamlessly. If one signal is 1/4 the way through the first field of the interlaced frame, and then you switch to another signal which is 3/4 way through the second field, well, there's no way a TV can handle that! And it may take several frames to lock onto the new signal for all I know, it's gonna be hardware-dependent. A TV and VCR will react differently. A 1990's TV set and a 2007 TV set and a plasma screen TV set will react differently.
AFAIK, the news stations fixed that by either running very special cameras with a single sync signal, or use a digital frame buffer to match up two dissimilar signals at the moment they switch. Without a common sync signal though, 2 sources won't even maintain the same phase relationship, they'll drift apart since one will always run faster than another.