I opened 2 projects and selected 2 items from one then copied and when pasting in to the other project it crashed Construct.
I just did what you said and it worked. Did you have both projects open in the same Construct window, or did you have two versions of Construct running in separate windows? Because you have to have only one copy of Construct running at once, something about memory.
Anyway, I don't think copying objects from one .cap to another is fully supported yet, I believe David is working on it. There does seem to be a bit of an issue. I selected everything in one layout to copy, and pasted it all into another .cap and it worked for the most part except the texture on my player (a ball) got pasted with a texture from another object (the ground). But when I copied the ball again from the first .cap and pasted it on it's own, it copied normally and the original I copied apparently updated itself too because the player/ground hybrid turned back into a ball . Weird.
I was able to copy all objects and all events from one project's layout to another project's layout and it worked fine. You still have to manually set things like layout and app properties, and create any globals or layers you need, etc.
But long story short, no you can't really do this yet. It's not an actual feature, more like a work in progress.
Also, apparently saving this copied .cap made a really funky .persist file, it made it so that I couldn't see the layout at all. When I deleted the .persist it opened normally. But yeah... you probably really shouldn't be doing this yet because it's not very stable.