What would you do if a residential customer wanted an new 120V dedicated circuit but panel is full?..... of tandems.

Pretend they don't want any added/upgraded panels.

Walk away.... fast!!!

Can you pull two circuits out of the panel and combine them into one circuit?

Subpanel ??? This is always a problem in older houses with a kitchen reno.
things like mircrowave hood combos dish washers that were hacked into the laundry plug in the basement, fridge relocate new counters that need more counter plugs now etc.

... Ya...

If you stay reallllly still, they might forget...
then you can steal all their cookies!

You can...

This isn't a real situation for me... just gettin some input.

It's a good idea, load permitting ofcourse.

Oh, its a hypothetical question. Run an extension cord to the neighbor's separate circuit outside plug. No one ever tripped over a hypothetical extension cord.

Actually, this problem comes up a lot. It's the reason the DIY stores sell SJ cord by the foot.

Ya it does come up a lot.

Can't really win either way... I mean it can be safe, especially if you fed the 2 circuits with a #12 (or #10 if it's an FP panel)
But running away is a little nicer for the pains.

Though I still raise an eyebrow when I dedicate a dining room plug. I can't remember anyone using it for something other than a lamp or phone, if anything.

I would say you can't answer this question without a load calculation.

Greg, in Canada we can put 12 outlets on a 15 amp circuit. When a panel is full, sometimes it's because every electrician ran a new feed for every new outlet. Then, we can pull two circuits out and combine them on one 15 amp breaker.

The dining room plug is outdated. I haven't seen a toaster on a dining room table for 30 years.

First look at the panel approval. I have seen panels approved for 24 circuits with 32 circuits coming out because the electrician installed 16 tandem breakers or some combination of full sized and tandems to exceed the approved number of branch circuits. IE a 24 circuit panel with 12 tandem breakers is full even if there are 12 spare positions left.
So in order for a brand new house to meet code there could be 10 tandems installed into a 24 circuit panel leaving 2 spare positions that may be used and a bunch that would create a violation.
Most modern panels are simpler in that if there were 24 possible full sized breaker positions the panel would also be approved for 48 1/2 sized breakers too.