questions, questions………

hello everyone. as you know, i have a 3.5 inch female carolinian. i have been considering finding her a friend (another female of about the same size) for her four foot 40 gallon tank. first, what are the odds that they won’t hurt each other? are db’s known to be vicious to one another? second, what should i do to avoid any squabbles? third, what extra expences should i expect besides vet bills? i buy food in bulk so i don’t think that should become too expensive. third, what is a good amount of tank space for two terrapins to cohabitate in? fourth, does anyone have a terrapin that meets my description for sale???