I've done something similar. Create a custom html file, call it twoplcs.htm or whatever, and place it in the same folder as the web visualization files.
In that file should be a couple of iframes, one for each PLC, which embed the two webvisu.htm files
<iframe src="webvisu.htm"></iframe> //local web visualization
<iframe src="plc2address/webvisu.htm"></iframe> //remote web visualization
Then you'll have to point directly to http://plc1address/twoplcs.htm
in order to call up your custom page instead of the default web visualizations