Using them together is incorrect, however, sometimes children will use them together to be silly (or adults behaving as children).

No, using them together is wrong

No. It is hot. It is hotter. It is the hottest (that) it has ever been. You COULD say: The weather is more hot than I remember. - - - But it doesn't sound quite right to a well educated native speaker. It is best to say: The weather is hotter than I remember. Part of the problem of listening to a foreign language that you are not yet fluent in is that you will not recognize when a native speaker is using a dialect or speaking badly or incorrectly. Foreign language teachers will almost always teach you a prestigious version of the language so that when you use it you are not identified by native speakers as someone who is poorly educated or using a regional dialect that is not well regarded.

The answers below talk about -er making adjectives into comparisons. As in your examples. . Obviously some words just end in "er". You can use some those, but they just mean different things. Sorry to bother you. I bring even more bother, this time.