useasboundingbox (0,0) rectangle (<width of first picture>,<height of first picture>);

documentclass{article}
usepackage{tikz}
begin{document}
fbox{%
begin{tikzpicture}
   draw (-1,-1) -- (5,5);
   % more drawing commands ...
   coordinate (FIRST NE) at (current bounding box.north east);
   coordinate (FIRST SW) at (current bounding box.south west);
end{tikzpicture}
}
fbox{%
begin{tikzpicture}
   useasboundingbox (FIRST SW) rectangle (FIRST NE);
   draw (0,0) -- (1,1);
end{tikzpicture}
}
end{document}

begin{tikzpicture}
% TikZ code for first picture
begin{scope}[xshift=5cm]
% TikZ code for second picture
end{scope}
end{tikzpicture}

newboxmybox
setboxmybox=hbox{begin{tikzpicture}
...
end{tikzpicture}}

wdmybox
htmybox
dpmybox

documentclass{article}
usepackage[most]{tcolorbox}
usepackage{array}
newtcolorbox{mysidebyside}[1][]{%
enhanced, sidebyside,
sidebyside align= center seam,
halign = center, halign lower = center,
#1
}

begin{document}
begin{mysidebyside}[]
begin{tikzpicture}
draw (0,0)--++(0:3cm)--++(60:3cm)--cycle;
end{tikzpicture}
tcblower
begin{tikzpicture}
draw (0,0)--++(0:1cm)--++(60:1cm)--cycle;
end{tikzpicture}
end{mysidebyside}

begin{mysidebyside}[empty]
begin{tikzpicture}
draw (0,0)--++(0:3cm)--++(60:3cm)--cycle;
end{tikzpicture}
tcblower
begin{tikzpicture}
draw (0,0)--++(0:1cm)--++(60:1cm)--cycle;
end{tikzpicture}
end{mysidebyside}

noindentbegin{tabular}{>{centeringarraybackslash}m{.45textwidth}>{centeringarraybackslash}m{.45textwidth}}
begin{tikzpicture}
draw (0,0)--++(0:3cm)--++(60:3cm)--cycle;
end{tikzpicture}
&
begin{tikzpicture}
draw (0,0)--++(0:1cm)--++(60:1cm)--cycle;
end{tikzpicture}
end{tabular}

end{document}