Managers are to emotional to decide when to use their one challenge or two challenges.
It works in the NFL because there is a team of coaches in the booth watching the replays and can buzz the manager to challenge a call.
There are no coaches with the ability to buzz a football/soccer manager and say challenge that one.
As an example, the call that didn't go Chile's way regarding the shove. The manager challenges, the ref looks , and the ref decides it's not a strong enough push to stop the player from going for his header, and decides he got the call right the first time.
Refs are not going to over rule themselves using VAR unless it really is a blatant thing they missed.
That's exactly what I want though: it's the blatant misses that need to be rectified, no need to revisit every incident with a degree of doubt.
If you leave it to the ref, the differing judgement calls, interpretation, and verging into stupid / sinister territory: not revisiting a decision that blatantly requires help due to being incompetent / in the pocket of some party, and ultimately, you have no control over the time used for it from game to game as it depends on each ref. The last bit could wreck the game as a spectacle.
If you give the calls to the watching manager instead of the guy running around all game, to be then decided say by the 4th official (qualified ref) once referred rather than the match referee, start conservatively with one each as dalarr suggested, if necessary go to two - you control the flow, the time used, you can aim for consistency. You know, at most, we'll like lose say 2 minutes a game, 1 minute a call, move on.
Ultimately, the target should be to correct blatant misjudgements imo. If you work to the target of wanting to check up anything with a degree of doubt, you'll completely wreck the game as a fluid spectacle. The fluidity is one of football's greatest assets, unlike a few other major sports.