If the two transformers are in the same yard or facility, then it generally a good idea to bond the grounds together to reduce problems with ground potential rise. This is generally required for installations in US covered by NEC.
Grounds that are not bonded together are still interconnected through the earth. A ground fault on one system could create a large voltage difference across the earth, and this can be a bad thing if you happen to have your legs too far apart.
Suggestion: The system grounding type of each transformer should be given appropriate consideration to bond the transformer grounding or not to bond the transformer grounding.
