Hi Eugene,
Note:
1) If you have added your 2nd SMP Server in CEM Gateway and you don't have any other remote Site Servers in this environment (such as Task Server or Package Server), then your clients in CEM mode will download packages from your SMP server and registered with your Task Server on SMP Server.
2) This is expected, that you cannot to add SMP servers as "Site Server" on CEM gateway.
Question:
1) What error message appears on client in CEM mode, which isn't able to download package from SMP server through CEM gateway?
2) Just to confirm: Your 2nd SMP server, which is added in CEM Gateway, doesn't have any other remote Site Servers and SMP server is a TS and PS in one time?
3) Did you properly configured settings on "Site Server Management" SMP console page? Site(s)/Subnet(s)/Assignments/Package Server codebases publishing?
Thanks,
IP.