I know that FAR 91.205 requires a rate of turn indicator for IFR (except for
airliners with three attitude indicators). Is it common for this requirement
to be waived to allow substituting a second AI for the TC?

In a manner of speaking. No waiver is actually necessary. Advisory
Circular 91-75 details the conditions under which this substitution is
acceptable. Note that 91.205 states:

no person may operate a powered civil aircraft with a standard category
U.S.
airworthiness certificate in any operation described in paragraphs (b)
through
(f) of this section unless that aircraft contains the instruments and
equipment
specified in those paragraphs (or FAA-approved equivalents)

91.75 Details what constitutes an FAA-approved equivalent for a
rate-of-turn indicator.

Michael