Join two tables without unique column (DAX)

You can create a calculated column in the first table with something like this:

= CALCULATE( COUNTROWS(Table2), FILTER(Table2, Table2[experiment_id] = Table1[experiment_id]) )

That will count all rows in Table2, where the current row's experiment_id equals the experiment_id in Table2.  If you want to sum the def_id, then change COUNTROWS() to SUM().