The HSYNC and VSYNC signals from the CRTC are not directly connected to the display. They are passed to the [[Gate Array]] for further modification. See its wiki page.
While When HSYNC and VSYNC are active, all the CRTC counters continue to increment normally and addresses continue to be generated.
On all CRTCs, while a VSYNC is ongoing, the condition VCC=R7 is ignored. So we cannot trigger a new VSYNC during a VSYNC. Similarly, we cannot trigger a new HSYNC during an HSYNC.