The advantage of multi-output (on some receivers) is the ability to set level and distance independently. If you have two subs placed at very different distance from your mains, it might be worth it, at lest in the long run. How much is it necessary right now (or even ever), I don't know, but I guess it depends a lot on positionning and if the subs are from different models or brands.
Subs experts are bound to join in soon on this topic!

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