I was going to comment much earlier. Then thought better of it. Both toilets acting up was somewhat signifigant.

I am all too familiar with the final output of lines meeting existing higher water levels. This gets the toilet paper left in the line to accumulate. As the flow reduces to zero. Then the toilets themselves seem to plug up much easier.

I found this was really problematic and intermittent on occasion so hard to establish. Typical on septic systems where the tanks fluid level increases because the field is too slow to drain. Usually because the people have increased the system loading periodically. I have had to improve the systems field capacity.

The so called ideal drain slope of historical standing is two percent or about a quarter of an inch per foot. Yet the coefficient of friction as materials used to manufacture drain pipes has changed. Has made me rethink this a little.

Since both toilets are acting up I would want to establish where the two lines meet anywhere before they discharge into the city system. Then examine beyond that. Looking for high water. Although as mentioned if this is intermittent it can be hard to find.

You might ask the city to examine their main line. Especially if it is a freebee. I suspect there is no regular need normally to clean or check it. Unless a possible complaint is received. Your plumbers seem to have found nothing really signifigant so far. If the main street line is older the city department may also have history on it.

If those two toilets where not problematic in the past something has changed. Although there was some at least partially hard obstruction removed in one line. It may or may not have been there for a long time.

People will be putting toilets out curbside for the spring cleanup soon. Enough of them each year with many being not that old. Things stuck in their traps is my probable assessment on many of them. Floating plastic toys or objects in the traps can drive the owners of them crazy.

You have a video that hopefully goes all the way out to the street. Try to examine the water level in the cities drain. Although your plumber should have caught it if was too high. Unless again it is an intermittent condition.

It probably would take your plumber to ask about the city drain if he finds nothing else. Two toilets not used by the same people increases the strength of my suspicion.

I have seen toilets operating on really far too flat drainage lines. They are slow but seem to not have major problems. All this said I am not a plumber at the same time.