As I understand what you are proposing, it can not be done. With more information on your panel make type and size, along with a clear understanding of your current and additional loads, a proper recommendation might be possible.

Can't give advice without more information; a photo of your breaker box would help. A "240V 20A" circuit breaker requires to adjacent positions with one giving one 120V line and the other giving the opposite 120V line, so the Voltage between them is 240V. Either line could also be used as a 120V line, usually for an associated circuit.

I didn't know 240V 50A slim breakers existed.

It may be possible to move some existing breakers around, but I can't tell from your description. It is also be possible to free up a spot by wire nutting two 15 amp circuits in the breaker box with a pigtail running to a single breaker. As stated above, providing a picture might get you better answers.

on the right in slots 8-9 is a 50a slim breaker; on the bottom left, I am trying to move the 20a breaker in slot 14 to slot 13, move the 20a breaker in slot 15 to slot 16, and install 50a slim breakers in slots 14-15; an electrician has told me that it can be done as there is a UIL-approved sonething that connecte the 2 non-adjacent breakers such that when a person turns one off then the other one is also turned off or when one breaker is tripped then the other breaker is also tripped; I cannot find the something

We still can't visualize it unless you post a picture. And it is not exactly clear to me what the electrician is saying from your explanation.

I tried uploading the photo 3 times and it did not work; here is my Dropbox link

https://www.dropbox.com/s/w6fu968zexy4uyi/20220325-082509_Tom.jpg?dl=0

You're really full! Following Kudzu9's suggestion, there don't seem to be any 15A circuits, and only two 20A ones! I can't exactly figure out the slot numbering system, but is there a half slot available in the upper right (slot 1?)? Is this the only panel in the house?

It looks from your photo that you have only two, non-adjacent single positions open, and all the other spaces are taken up with tandem breakers. Is that correct? If so, the only solution I see is to pigtail two pairs of circuits. You can take two of the 30 amp tandem breakers out, pigtail the wires correctly, and connect the pigtails to one of the 30 amp tandem breakers. That will free up two more spaces. Would that give you what you need? Here is an article that cites this approach as allowed by code: Pigtail in a breaker box

I believe there are 4 open single slots (top is 1) left 13, left 16, right 1, and right 12; left: 1-4 go to subpanel in garage that has all the 110v circuits, 5-8 furnace, 9-12 furnace, 13 unused, 14-15 Egague (solar monitor), 16 unused; right: 1 unused, 2-3 water heater, 4-5 dryer, 6-7 AC, 8-9 oven, 10-11 master bath water heater, 12 unused, 13-16 solar

Well, if you have 4 single slots, why don't you just shift breakers? For example, moving the 3 tandem ones on the left up will give you a pair of open slots right above the 20 amp tandem at the bottom left.

And it's great that you posted a picture, but without showing the numbering for the slots also, it's quite challenging trying to figure out the numbering from your description. It may be clear to you, but we're not there.... ;-)

there are 16 slots on the left and 16 on the right; I numbered them 1 at the top down to 16 at the bottom

Ok...so you've got 4 free single slots?

yes

Thomas-

So the solution seems simple. Just pull and shift the existing double breakers one-by-one so that the 4 individual slots become 2 pairs of adjacent, double slots. Of course, for safety, turn off the main breaker, but pulling and re-seating breakers is simple and should take all of 5 minutes. And no rewiring is required. On the left side, it looks like you can just move up the bottom breaker one space and you'll then have a double slot space at the bottom; on the right side, you can move the top five breakers up one space and then you will have a free double space above the 60 amp breaker at the bottom (or you can also move the 60 amp breaker up also and have the new double slot space at the bottom). Does this make sense?

Tom-

Did you get this resolved?

it makes sense; thanks

Good luck. Re-post if you run into further issues.