I've created and set up a (virtual) domain with several VMs, all using a single NIC on NAT each, inside/under the same Workstation. I don't think two NICs are needed to set up such an "infrastructure", even though I remember seeings some posts online - not necessarily here - talking about using two NICs. In the meantime, however, there're more posts - even in this community - talking about using only one NAT NIC on each guest. I believe you can do the same.
Besides this, I believe "shared folder" between the host and a guest doesn't depend on the type of VMnet. Or, at least, "shared folder" is possible with NAT. However, I'm not an expert in this. Nor have I tried or used "shared folder" myself. So, please take this statement with a grain of salt.
My point is that, if we/you can break down what you want to achieve, I don't know if you have to have two NICs on each guest. As you said, a different IP on the 2nd NIC might have caused a problem. Perhaps that's indeed the case.
In my experience, I did try to set up two NICs on each of my guests when I tried to build my virtual domain. It didn't work out for me. But, I didn't go back to try a "negative" test case to confirm or find out if two NICs, as how I set them up, really caused the whole thing not working, after I had made one-NIC set up work, which is actually pretty simple and easy. So, again, I suggest using one NIC on NAT, if you don't have to have two.