Alex + Benedetta doesn't stack.
I can also confirm that Teresia + Alex doesn't stack either.
I can also confirm that the first Atk Boost (Male) you deploy stays in effect while the second one is negated.
For example, if I deploy Teresia first and then Alex, Atk Boost (Male) Lv4 will not override Lv3 and the stats will stay at the levels before Alex was deployed.
It's the same result if I deploy Alex first and then Teresia. Atk Boost (Male) Lv4 will stay in effect while Lv3 is ignored.