I would have said that if you have two identical transformers in parallel, with the primaries in parallel as well, that would usually be OK. There might be a problem with large transformers as they have better regulation, which means that their output drops less at full load.
If you are rectifying the output of the transformers, you can always put a rectifier on each transformer and parallel the outputs of the rectifiers. There will be no problem doing that.
It is certainly common practice to parallel two secondary windings on one transformer.