An actual video mixer may be your only option for that.
http://www.pssl.com/bitemdetail.tpl~waGroup1datarq~[waGroup1datarq]~eqint_KeyIDdata~44960~itemNumber~AVM01%20%20%20%20%20%20%20%20%20%20%20%20~UID~2007102514585815~disp_IMG~Y~catgroup~%5BcatGroup%5D
They aren't cheap, however they do have more options than the VP unit @ $400.