How should I allocated SLCSP percentages?

It is up to you. Even though your depedent will not file taxes, he will need to show coverage on your tax return. The easiest way is 33% for each, but depending how much you other child earned, he may not need that much allocation.

You can indicate that you shared the policy and prorate. See the screen shots below.  The pro-rations must add up to 100%. You can also click on the blue link.  What percentage of the following policy amounts do you want to claim on your tax return?  See "How do I figure out these percentages?" when you get to that screen.

Child/Children on another tax return

If you share a policy with someone who is claimed on another tax return (usually a child), you can take the number of people on your tax return who are covered on that plan and divide it by the total number of people on the plan. This would be your percentage of premiums, SLCSP and advanced payment of the premium tax credit, unless you agreed on a different number beforehand.

Can I assign 33% to each of us, even though one will not file taxes?

Yes. You are allocating how much of the insurance covered each of you. You don't have to do it this way. You could divide 50/50 between you and your non-dependent, but it may hurt you.