Um... one way would be to use check_http from one nagios server against the tac.cgi page (Tactical Overview) on the other, and use the "-s" option to check for an expected string in the content, be that something like "0 Critical" or, depending on how complex you want to get, "123 OK"* for services, and similarly, "0 Down" or "123 Up"* for hosts...
HTH
/S
** where 123 would be your total services/hosts that are being checked - of course, every time you add or remove a service/host you would then have to change such a check... I suppose you could get around this by writing a custom script using something like Perl and LWP useragent to download the tac.cgi webpage and search out the "ABC OK" or "DEF Up" values and compare them against the "Active Host/Service Checks UVW / XYZ" value, and alert if ABC or DEF, shouldn't be to difficult to set up if you know your way around Perl or similar*
