It is not okay to have two earth grounds in this instance. There can easily be a difference in electrical potential between the two. Differing ground potentials can cause some flaky effects. It is indeed against code here in Northeast USA and probably in most of the country since most codes are 'standardized' now.
Your outlying observatory box should be such that the neutral and ground busses are separate. These are then combined back in the main box. Some subpanels have busses that are made to be separated, some do not. Just check before you buy.

dan