Are you talking about carrying a slung rifle, or just holding it in your hands at the ready or port arms, because those are 2 different things. A sling takes the majority of the weight and puts it on your shoulder and back. But enough with semantics, I think the one weapon rule is fine. Can you draw both weapons before combat when you anticipate danger? Of course. Should you be able to draw 2 when surprised for free, or when combat starts? No. Why, because it gives you a reason to invest in the dual-wielder feat. You can survive a round of combat without your bonus attack. But really it's up to yiiur DM how they want to handle the situation. The majority of the time, if you sense you're going to be attacked, there's no problem readying weapons.Holding a weapon in hand is not as hard as all that. Granted I only have experience with a rifle or bow. But have carried them for hours and days on end without any issues...
That being said I don't see any problem with drawing two weapons with one free interaction, if ones normal weapon load-out is two weapons then one should be practiced in that action. This game is played with a DM for a reason, to allow for things not spelled out in the rules.
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