#solphim

The short answer is that it's up to your opponent.

When multiple replacement effects want to modify the same event (in this case damage being dealt by a red sou

When multiple replacement effects try to modify the same event, e.g. damage from your Lightning Bolt to your opponent or their creature/planeswalker, and none of those effects take precedence, then the affected player gets to choose which one to apply first.

In this case, the affected player is the opponent being dealt damage, or the opponent who controls the permanent that's being dealt damage. So they get their choice of adding then doubling or vice versa. This means that they'll likely opt to double first since that results in less damage to them or their creature/planeswalker.

When I answered a similar question recently, there were several comments wondering why the effects' controller didn't get to choose. So why does it work this way? A couple of reasons. First, there could be a situation where different players control the effects and disagree on the order in which they should be applied, in which case letting the controller of the effects choose just doesn't work. Additionally, this is how multiple replacement effects are handled for everything in the game and carving out an exception for one case involving less than 50 cards just isn't necessary.