%Formelsammlung_Main
%
%Präambel:
documentclass[a4paper, 12pt]{article}
usepackage{nccmath}
usepackage[export]{adjustbox}
%
% Custom Commands:
newcommand{newTab}[1]
{
    begin{fleqn}
        begin{align*}
        begin{array}{|*{18}{l}|}
        hline
        #1
        end{array}
        end{align*}
    end{fleqn}
}
%
newcommand{tabNoLine}[3][6]{multicolumn{#1}{|p{5mm * #1}|}{#2} & multicolumn{18-#1} {|p{5mm*(18 - #1)}|} {#3} \ hline}
%
%
% Document
begin{document}
%
newTab
{
tabNoLine[8] {Left (4cm)}{right (5cm)}
tabNoLine[10]{Left (5cm)}{right (4cm)}
tabNoLine     {Left (3cm)}{right (6cm)} % Default argument (6)
%
%
% I want to create someting like this using the custom commands:
%
%multicolumn{8}{|p{40mm}} {Left (4cm)} & multicolumn{10}{|p{50mm}|} {right (5cm)} \ hline
%multicolumn{10}{|p{50mm}|} {Left (5cm)} & multicolumn{8}{|p{40mm}|} {right (4cm)} \ hline
%multicolumn{6}{|p{30mm}|} {Left (3cm)} & multicolumn{12}{|p{60mm}|} {right (6cm)} \ hline
}
%
end{document}