I believe I was not seeing anything prior to GSX-2 live update to the system.
Of course you couldn't see it: we had no custom jetways before, and no jetway numbers. Jetway numbers use the RTT (render to texture) option, which your system is apparently not compatible with in SLI mode. Of course, you couldn't see any flickering on something that wasn't even there before (jetway numbers)...
Is there anyway I can force install GSX-1 only and maybe older CYVR & KJFK and see on what is happening. .
No need to do that. The "Disable RTT" option of the Addon Manager does exactly that: disables the Render to texture option, in case your system is not compatible with it.
Of course, you won't see jetway numbers anymore but, at least, you'll still have all the rest of the GSX Level 2 features.
I never had any SLI problems since P3Dv3.4 onwards
No other addon probably uses the new SDK to its fullest.