The performance benefit from an index for a particular query depends on the search parameters (aka where clause).
A single index with object_type and object_id together (and in this order) would help queries that specify object_type; or object_type and object_id in the where clause. It would not help if object_type is not provided.
Two indexes - one on object_type, other on object_id, would individually help based on the existence of the corresponding column in the where clause. If a where clause specifies both object_type and object_id, specificity of values would determine which of the two indexes are picked up for your query.