Can you separately text 2 different numbers under the same contact name?

ex: i have a contact named "mom & dad". i have both their separate cell numbers within that same contact. i was texting with mom and wanted to text dad happy birthday. i backed out of mom's, selected new text, went to mom & dad contact, selected dad's number. it opened what appeared to be new text thread. typed happy birthday and such. clicked send, and the ongoing thread with mom opened up with my new happy birthday showing? at first i didn't know if maybe it went to him but just showed up cause it was under the same contact name. but then i wondered how could you tell who you were texting if they both were under the same thread. it wasn't like a different color or said "dad" at the top of that sentence and "mom" on the rest. i guess that would.be too sophisticated to have a little mom or dad over who was sending what. anyway after asking who got what, turns out it went to mom and not dad. i have them both under the same contact obviously cause they are both my parents. they each have their own number so i wasn't trying to create a separate text with the same number, which i read iphone can't do. i wanted to keep them both under the same contact name to make it easy. when i use the phone to call it will tell me who was last, so it seems like it should be able to keep them apart. do i have to create another contact to be able to text both at the same time with different threads ?