Sendak’s Lightning Lash should not be Twinnable
For several reasons:
The range of similar spells is Self
Sendak’s Lightning Lash gives the caster an effect and attacks another creature. In practice, the spell has two targets. There are many other spells that behave in this way, and they all have a range of self. See for example Produce Flame, Flame Blade, Shadow Blade, and Vampiric Touch. The design intent is that a range of self overrides the other range, so Sendak’s Lightning Lash should also have a range of self and should not be twinnable.
The behavior of weapons is incompatible with twinning
Sendak’s Lightning Lash explicitly describes that the arc weapon "deals 2d4 lightning damage on [a] hit", which means that everything the caster hits with the arc weapon is also a target of the Sendak’s Lightning Lash. Likewise, anyone who suffers a Dragon's Breath is also a target of Dragon's Breath. Therefore, Sendak’s Lightning Lash can target more than one creature during its duration by making melee weapon attacks against differing creatures. This means that it can't be twinned, just like Bigby's Hand, Telekinesis, and Maximilian's Earthen Grasp.
Twinned spells should not affect one creature twice.
The primary reasons why Twinned Spell has the targeting restrictions that it has, is to ensure that a twinned spell cannot be redirrected to effect one creature twice. Moreover, it is the logical conclusion of the rulings I linked above for Dragon's Breath and Maximilian's Earthen Grasp. Your intended behavior for a twinned Sendak’s Lightning Lash goes against this intent in two distinct ways:
- The caster receives a double buff in the form of two arc weapons.
- A single target can be attacked by both arc weapons.
It may be possible to very carefully and artificially word Sendak’s Lightning Lash to get around these problems, but then you are effectively designing a spell around a metamagic and your spell will forever stick out like a sore thumb among every other sorcerer spell.
If you really want Sendak’s Lightning Lash to support two weapon fighting, I recommend just giving the spell that functionality, instead of relying on some janky interaction with Twinned Spell.