I am unable to use two different fonts to create a custom decal. System allows me to use only one type of font.
For example I want use Arial and Calibri both for creating the decal. Can I do it? If so how?

You'll need a fairly sophisticated graphics program (ex: Photoshop, Affinity Photo (on Mac) or even GIMP), and use the FONT tool (in the decal design) and save it as an image (since using the font directly is illegal, since you don't own the rights to distribute the font directly).

Doing that, you can kern the letters individually. I do that whenever I find a font I really want, but it's missing the necessary characters (basic punctuation marks). Purchasing the font generally gives you the use to USE the font professionally, but not to modify the font. Though, if you mix fonts, you'll also need to not only credit both font designers, but also list yourself if you 'muck' with the characters (just so no one assumes what they see is either of the actual fonts).