Spiritual weapon does not continue attacking automatically. It attacks again when sustained. Sustain a Spell is an action. This also replaces the concept of 'concentration' as a duration for spells. Spells either last for a specific amount of time, or last for a duration of 'sustained' which means that the duration will extend to the end of the next round each time that it is sustained.
You can sustain the spell multiple times in the same round in order to have the Spiritual Weapon attack multiple times. Doing so does not extend the duration of the spell any more than sustaining it once does. Each time it is sustained it sets the expiration time of the spell to the end of the next round.
You are also able to cast a second Spiritual Weapon spell if you feel the desire. I fail to find a scenario where that would be beneficial though. The second instance of the spell would still be using your Multiple Attack Penalty, so attacking with them both on the same round will still be subject to increasing penalties for the additional attacks. And you could just as easily attack with just the first instance against multiple targets - switching targets each time the spell is sustained.
we have someone who is deleting their question after it was answeredYes?
Or do we have a new forum wonder - an answer without a question at all? (Instead of chronomancy)
Indeed. The question was something along the lines of: can we cast multiple instances of Spiritual Weapon. It also had some edition confusion with D&D 5e such as concentration duration of the spell and thinking that the spell would continue attacking each round without any further cost.