One plant much smaller than the other

Hi everyone,
About two weeks ago I germinated and then planted two seeds in two pots.

Unfortunately, when I planted them, I was kinda... intoxicated. And like the crazy dumbass that I am I ended up planting both in the same pot. Oops !

I transferred one pretty carefully into the other pot once it'd started growing and I realised my mistake.

Now, unfortunately, it's a midget. It seems healthy, just it's some sort of Verne Troyer-ish version of the other one, which seems to be doing fine.

Attached are two pictures of each plant - the larger, healthy one, and the smaller, runty one.

They're both being grown in identical conditions in a well-lit (was 20/4, now 18/6), well-ventilated closet and being watered once every three days, which is basically just whenever the soil is dry enough to need it.

Is there anything else I might be doing wrong?

Also, is there anything I can do to get Stuntie (the shorter one) a bit healthier?

Thanks!

JB