SIMPLE IRA & Mid-Year SH 401(k) - Separate Plan Sponsors

I'd love the thinking of folks who are more well-versed in SIMPLEs. Didn't get any traction on the other board, so I thought I'd try the 401(k) board since it does involve one.

Company A maintains a SIMPLE IRA in 2024 & 2025; several employees of Company A create their own Company B in 2025. Company A maintained a SIMPLE IRA (I'm not sure if the SIMPLE IRA is still active); Company B established a 06/01/25 effective date SH 401(k) Plan (short initial Plan Year). Company A & B have different EINS with no ownership crossover.

I understand that when an employer establishes a mid-year 401(k) Plan that the deferral limit is adjusted based on the # of days/365 of each arrangement. Because these are two unrelated employers, my thinking is that this does not apply to this scenario, so all EEs can contribute the total $23.5k between the two arrangements if they would like (a maximum of $16.5k being attributable to the SIMPLE IRA).

• Do you agree that the deferral limit for the Company B 401(k) Plan does not need to be pro-rated based on the number of days it was in existence vs. the SIMPLE IRA?
• Since catch-up contributions are separate to each Plan, can a 50+ participant who contributed $10k to the SIMPLE IRA under Company A defer an additional $24.5k to the Company B 401(k) Plan? ($6.5k SIMPLE deferrals, $3.5k SIMPLE catch-up, $17k 401(k) deferrals, $7.5k 401(k) catch-up)
• If the Company B employees are still employees of Company A and participating in the Company A SIMPLE IRA, does that matter? Or is it just a consideration in that both the non-catch up deferrals to each arrangement count towards their overall 402(g) limit?
• As I write this out, I imagine that a relevant consideration is whether Company B and Company A constitute an ASG. If they do, would their contributions be subject to the adjusted deferral limits based on the days/365 of each arrangement?

For the ASG question - IF they are one - there is a whole other issue of not having a SIMPLE at the same time as a 401(k) plan, by the same employer. And an ASG is treated as a single employer for those purposes, so generally cannot have both in the same year. Determining the status of the SIMPLE would be very important. 

The deferral limits for short initial year 401(k) plans generally aren't pro-rated as they are personal limits, not plan limits, but the plan document should address if there is any pro-ration of limits (deferral or otherwise) for an initial short plan year where there is no prior SIMPLE or predecessor plan. If there is a basic plan document for the 401(k) plan, you should read it carefully.