connecting the negative positive terminals of two dc to dc converter
No, this works fine providing the outputs are isolated types. I've done this a few times. You can even add multiple outputs to get oddball voltages. For instance I used a 30V output and a 3V3 output but subtracted the outputs to get 26.7 volts.

Here's a picture of two devices powered from a common supply but producing negative and positive 5V: -



Just read the small print to make sure the outputs are isolated. Here's a stack exchange post that does the same.