For anyone else reading, you would make your life a whole lot simpler if you used one catalog, not just for this reason but for dozens of other reasons as well.
If the method of saving as a catalog isn't working for you (I have never tried it), then I'm afraid it probably can't be done easily. The only method I can think of is to manually re-create the collection in the 2nd catalog.
But that seems to work only if the photos are not already present in the other catalog, otherwise nothing is loaded.
I just verified that this method worked for me in LR 7.1 when the destination catalog already contained the photos in the collection. My steps:
1. In the source catalog, I right-clicked the desired collection and did Export This Collection As A Catalog. I unchecked the option Export Negative Files.
2. In the destination catalog, I did File > Import From Another Catalog and selected the catalog exported in step 1. I set the option Replace: Metadata And Develop Settings Only.
thanks for trying. I try the same in LR CLassic and before in CC and I have no success. I do write sidecars and there may be another Setting in preferences that let me not allow to do that.
I see your determination to advice everybody who slightly tends towards multiple catalogs not to do that. I am then just not very gifted in handling software. I have >20 catalogs, one for each year. Each has 20-50 collections and nested sets and and and. If I throw all that in one catalog I have a tree of 20*50 collections and more. This may be a good thing for archiving data (and that is what I do). But not for working with it. Simply collapsing and expanding such a tree takes ages. Simply manipulation that tree takes ages. Imagine adding 2018 to that tree now with all the needed DAM manipulation: Pulling in iPhone photos and sequencing them with the others, re-date WhatsApp photos, include clips and battle with their date, export photos to DXO and get them back in the PL catalog that sits somewhere in that 21*50 tree and pulling then these photos from there into a collection that feels like sitting 3 meter lower... This would be an option if LR would hide non-needed collections.