1. Can I use one HDMI transmitter ADV7511W connected to two monitors?
2. Is there a solution to isolate (2.5 kV) between HDMI transmitter (~75 MHz) and connector HDMI RX?
1) No, HDMI was designed to be a point to point link. Even disregarding HDCP and EDID precluding this, you'd still need to buffer to do it.
2) Assuming your system never has to pass compliance testing, you should be able to AC couple but there isn't a turn key solution for it. The HDMI spec mentions AC coupling on the RX side, but not on the TX side.
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