Sometimes, but they come from different environments with different water chemistry, so technically no.
Platies come from water with a GH around 200ppm and a pH above 7.0.
Siamese fighting fish come from water with a GH below 150ppm and a pH below 7.0.
Some male Betta splendens (Siamese fighting fish) are quite peaceful and get along well with other fish, but others can be real nasty and go nuts and attack any fish they see.
If you want to try keeping them together, you need to get the GH around 175ppm and keep the pH close to 7.0, and monitor them closely. If the male Betta shows any aggression towards the platies, you have to separate them.