Say, if the specs are at 5 V and 25 °C and I have to use the part at 7.5 V and 50 °C and I have two separate curves for voltage and temperature, how can I combine them?

Every product is different -- so there isn't an answer that will fit every case.   You can call our tech support operation or send an email through the contacts on the web site about the product you are interested in.