Is concomitant use of azathioprine and febuxostat safe from a bone marrow toxicity standpoint?

No- Logan et al., PMID 31885095.

See evidence. If you must use both for outside reasons- start really low and slow and monitor very very closely!

There is more reason to consider a transition from azathioprine to another agent, such as mycophenolate than to take the risk of myelosuppression.

Mycophenolate is safe in combination with febuxostat.

Generally, I would avoid using both allopurinol (and/or febuxostat) and azathioprine (Imuran) together. The risk of course is bone marrow aplasia, as we know.

No, the use of the two in combination is contraindicated.