It only makes sense that the HDMI source can send out only ONE type of signal at a time. Since most higher-def TV's can handle a lower-def signal (e.g. your 1080P set can handle a 720P signal OK) but NOT the other way around, then it is necessary that you feed one 720P signal through a splitter to both TV's. Now, I'm not so sure that the HDMI system and the TV's are smart enough to figure out which signal type a TV requires. I suspect YOU have to set the HDMI source to the 720P setting. If you set to 1080P, I expect the less-capable TV will display nothing.
IF you are feeding two different TV's from a computer graphics card, MAYBE there's another way, too. IF the graphics card has dual HDMI outputs that con be configured separately...