There is a way to achieve your expected result by playing around with the grouping settings.
First create a tablix and right click on the second cell of the first column.

Select Add Group, Row Group - Parent Group. Then just use the following settings.

Be sure to check Add Group Header checkbox.
Now select the recently created group column (the first) and delete it. Note the groups will remain.

Once you delete the column you will have two rows in one group. Just use the exprressions and set the bold format for the Group
cell.

It will produce the following output.

Let me know if this helps.