Can a Backup Generator SubPanel Neutral be tied to the Main Panels Neutral bus, if both panels are energized at the same time from different sources?
We see a lot of really horrid transfer switches on this forum, believe you me.

The Backup Generator Subpanel (Siemens - gray panel) is my &quot;economic&quot; version of a Transfer Switch, with a mechanical interlock between the upper &quot;30A generator&quot; breaker and the lower &quot;30A subpanel feed&quot; breaker.

What you did there is absolutely excellent.  This is one of the best generator subpanels I've seen.  Now there are some tune-ups we can do to improve it; some trivial and some rather critical, but those are only because you understood how those hokey $350/$450/$550 6/8/10 circuit &quot;transfer switches&quot; work, and copied too much from their design, which is a bad design. **It isn't a matter of &quot;economic&quot;, what you did here is actually a much better design than what you were trying to imitate.   Honestly, this is what we recommend people do, and they rarely listen lol.
Tidying it up
What you actually do have there is a subpanel.  So it needs to be wired like a subpanel, which will dictate a few changes.  Other than that most of these are just recommendations.
Mandatory: Must bring neutrals over.  When you bring a circuit over from another panel like this, you are required to bring over both hot and neutral.  It's OK to leave ground behind, in fact, since you wisely used a metal conduit nipple, that takes care of grounding.

&quot;But transfer switches don't do it that way&quot; - yeah... I know they don't... and it boggles my mind, because it's contrary to all modern Code practices. Not least, those hokey transfer switches break AFCI/GFCI! Your method can support them in both utility and gen mode.

Mandatory: the neutrals will require fatter/more conduit. If any of the 1/2-3/4&quot; conduits above align, feel free to add some.  Otherwise knockout the lower conduit to the biggest hole both panels can take (be careful not to over-knock; I hate adapter rings) and fit that size of nipple.
Mandatory: Use THHN wire for the interconnects.  You just took Romex and shucked the sheath off of it to get individual wires. That's fine for pigtails but this work requires real THHN.  Also: Hot can't be white.  Hots must be other than white, gray or green.  That's because you're working in individual wires; remarking a white wire to be a hot is only allowed in cables.

You're also required to group or mark hot-neutral pairs; but you have that covered.

Mandatory: Backfeed breakers need tie-down so they can't be rocked out by accident.  That'd be the two breakers on the interlock. You can buy tie-down kits, or since you're in the Siemens world, the ECSBPK01 interlock inherently provides that tiedown by placing the breakers at the top across from each other.  It's $27.
Suggestion: Upsize the utility-side feed.  It can be any size you like up to wire limits (rounding up). Remember THHN uses the 75C ampacity rating, so #8=50A..... #6=65A (use 70A breaker).... #3=100A... #2=115A (round up to 125A).
Suggestion: Upsize the panel. While you're in there, think about a panel swap to a much larger panel, e.g. 30 space.  Seriously, pretty much our slogan on this stack is &quot;Go BIG or go home&quot; - spaces are dirt cheap, and running out of spaces really sucks.  As said, there's no trouble whatsoever sending as much as 125A over to this panel, so you can put any circuit you please over here, you're not limited to 6/8/10 like those transfer switches.  You're also able to use AFCI or GFCI on this type of setup (once neutrals come over), but those are full-space breakers so you need lots of space.
When I'm mocking the 6/8/10 circuit transfer switches, I typically choose a 24-space panel at around $55 just to show people they can have 3 times the spaces for 1/3 the money lol.
Suggestion: Blank cover plates.  I hate them, I find them fragile, hokey and dangerous. I just fit real breakers there and just stick red electrical tape across them to indicate they are unused.
No alien breakers - you have Siemens breakers in the Siemens box and Square D breakers in the HOM box.  Well done.  By the way if you have need for a &quot;universal&quot; breaker that fits both, Eaton CL series is UL-approved for both panels, and they do come in AFCI/GFCI (1-pole only). Do not use Eaton BR.
Wiring up inlet for 240V + using 120V adapter cable is excellent and the correct way to do it.  That way on the happy day you get a 240V generator, it'll just plug in.  You are on top of the MWBC issue.