Haste lets you make attack rolls that can affect more than one creature before the spell expires. So haste is not a valid spell for twinning.
How many concentration spells can you have active 5e?
No, you can only concentrate on a single spell that requires concentration at a time. You can have more than one spell active at a time, but a single character can not have two or more concentration spells active at once.
Can you hold 2 concentration spells?
There are no rules for letting you maintain concentration on two spells at once. There are features that can let you cast a spell without needing it to have concentration, some spells can let you avoid having to maintain concentration to sustain the spell if cast at higher levels (such as Major Image).
Can you cast a new spell while concentrating?
There are only two cases where casting a new spell will interrupt concentration of a previously cast spell (that I can think of off the top of my head). The new spell is a concentration spell. As soon as you start casting it – the concentration on the previous spell will break.
Can you cast multiple spells at the same time?
Yes, you can cast multiple spells at the same time and have both take effect. There are no rules against this. The rules are fairly clear about what is and is not allowed with regards to when and how spells can be cast.
Can you cast sacred flame while concentrating on a spell?
So on turn three you could maintain Concentration on a Bless, cast Sacred Flame as an Action, and then either use the already cast Spiritual Weapon or cast an entirely different Bonus Action spell (as long as it does not require concentration) and still not lose the Bless or Spiritual Weapon so it could be used in subsequent turns. Was that clear?
What happens if you lose concentration on a spell?
Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends. So what happens if, for example, you are already concentrating on a spell, and cast Witch Bolt without concentrating. Would you get the initial arc of blue energy, or would it not even go off?
