Eligibility Requirements of age 21 and 2 years of service...requires dual entry dates?

IRC Section 410(a)(4)

(4) Time of participation
A plan shall be treated as not meeting the requirements of paragraph (1) unless it provides that any employee who has satisfied the minimum age and service requirements specified in such paragraph, and who is otherwise entitled to participate in the plan, commences participation in the plan no later than the earlier of—
(A) the first day of the first plan year beginning after the date on which such employee satisfied such requirements, or
(B) the date 6 months after the date on which he satisfied such requirements,
unless such employee was separated from the service before the date referred to in subparagraph (A) or (B), whichever is applicable.

What WDIK said; also see Treas Reg § 1.410(a)-4(b)(1).

What is often different in plan design when 2 years of service is required rather than 1 are two items. One is the measurement period for eligibility years of service. The first one must be measured as of the first anniversary of date of hire. Succeeding ones may be measured between anniversaries, or "shift" to plan years beginning after the date of hire.

The shift is common with plans requiring one eligibility year, but can be counterproductive with those that require two. For example, consider a calendar plan year that requires two eligibility years, but also shifts to the plan year for measurement. An employee hired 12/29/2007 actually enters the plan 1/1/2009. He earned one eligibility year by 12/28/2007 if he had 1,000 hours of service from 12/29/2007-12/28/2008. He'd earn the second required eligibility year on 12/31/2008 if during 2008 he had 1,000 hours of service. 1/1/2009 being the first day of a plan year, it must be an entry date.

Had the shift not been elected, the second eligibility year could not have been earned before 12/28/2009 and entry not before 1/1/2010.

So even though you might have given up a vesting schedule to be able to require two eligibility years, you can lose much of the benefit of it by also designing in the shift.

The other difference is that if the plan also sports a 401k feature, you can only require 1 eligibility year for entry into the 401k feature, although imposing 2 eligibility year requirement for company contributions.

No, dual entry not required. "Back in the day" before 401(k)s ruled, we had quite a few plans with 2 year eligibility and nearest Jan 1 (or first day of the fiscal year) entry. And as J Simmons notes, we measured eligibility on what I call the "employment year" - i.e. from date of hire, not switching to the plan year after the first year.

By dual entry dates, are you asking if there would be one entry date into the plan's 401k feature (if it has one) and a later entry date into the plan's profit sharing feature? If so, yes.

Otherwise, no.

Frankly, dual entry dates are never required for straight MPPP or PSP if you are agreeable to retroactive participation to an earlier date for 1st half of year hires (i.e., Bird's "nearest Jan 1" possibility).

But then you have another issue, not unlike your eligibility computation period issue: you'd always have to use full-year comp for allocation purposes if you had a single entry date, because entry would always be at the start of a plan year, rather than having some participants enter in mid-year and including only comp from that mid-year entry date.