This works, but may not look the way you want...
Create your two parameters and allow both to have blank or null values.
Create a Dataset, with this Query:
select case when nullif(@p1,'') is not null or nullif(@p2,'') is not null then 'Valid' end as result
Map the parameters in the dataset @p1 to Parameter 1 and @p2 to Parameter 2.
Create a third report parameter which does not allow blank or null values.
Set its available values to be sourced from a dataset and choose the one you just created.
Set its default value to be sourced from the same dataset.
The dataset will only contain a value when at least one of the main two parameters has a value, preventing the report from running (parameter 3 requires a value) until either of the first two parameters is populated.
You can give the 3rd parameter a prompt, such as "Please enter at least one value" if you like.
MM
select geometry::STGeomFromWKB(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