If you really want to go the dual GPU route, I'll already assume that you've checked SLI compatibility against the list of games you play and you are happy that it'll be worthwhile in terms of support.
Something you need to consider is SLI scalability. Even when SLI is supported, if the scaling is less than 50% (for example) then you might not hit that consistent 144fps. At this point I'd say you have a few options...
1. Get dual 1080Ti's
2. Get a G-Sync compatible display
3. Lower graphics settings a little bit more than normal
4. A combination of the above
It's a tough ask in the more demanding games, even for dual 1080s - as you are looking to get 144 fps as a minimum, not as an average.
There are GTX 1080/1440p SLI benchmarks with 1% lows/minimum fps online that'll give you an idea of what you are up against.
Additionally, if you look for the minimum fps/1% lows of the GTX 1080 in games at 1440p, then try and find the % scaling observed by users online, you can work it out from there.
I'm running OC-ed bios modded dual R9 Furys with a 144Hz 1440p Freesync display. I cannot get a locked 144fps in all games either, because of Crossfire scaling or the games are just too demanding at Ultra. You will get closer with the 1080's ofc, but a G-Sync monitor is a great way of ensuring smoothness when you get those inevitable drops below 144fps.