Issues when other DC plan has a different plan year?

[John A]

When a plan sponsor maintains a 401(k) plan, and also maintains another DC plan with a different plan year than the 401(k) plan, what special issues must be considered?

How are 415 limits determined (if the limitation year for each plan is the plan year of that plan)?

How is 404 deductibility determined?

How is 410(B) coverage determined?

I am aware that for 416 top-heavy purposes, the plans are aggregated by using the determination dates that fall within the same calendar year.

To determine ADP and ACP tests [if the 2nd plan is also a 401(k)] the plan years ending within the same calendar year would be added.

To determine who is an HCE, plan years beginning in the same calendar year would be aggregated.

Are there other issues that require aggregation of the plans?