I have both USA region user, and Saudi Arabia region user, both on same playstation console,
Can I subscribe to the PS plus America account, and still be able to use the Saudi Region User to play online game
Yes as long as the game you have purchased isn't region restricted for play. You probably will not know this until after you buy the game and try to play it from the other account though.
I have both US and JPN accounts on my systems and play the Japanese titles from the US account all the time (and visa-verse).
Maybe this answer your question: I have two Playstation users, one for Switzerland and one for the US. I have PlaystationPlus on my Swiss account but I don't have the benefits of PS+ on my American one. But the thing is, I have two PlaystationNetwork accounts for each Playstation user. Do both your Saudi Arabian and American user access the same PlaystationNetwork account? If you so, then it should be possible. If each user has a separate PSN account it will not work.