The behavior you observe is due to the formatting rules associated with Times. Please start by reading my answer here: Returning an unevaluated expression with values substituted in. We can apply a similar technique here though the result is not quite as desired if we merely block Times during Box creation. We get:
$\left(1*\frac{1}{2}\right)*\left(3*\frac{1}{4}\right)$
This form is due to the internal format of 1/2 and 3/4:
Hold[1/2, 3/4] // FullForm
Hold[Times[1, Power[2, -1]], Times[3, Power[4, -1]]]
One way to handle this is to post-process the Box form yield the format we desire:
SetAttributes[hf, HoldAll]
MakeBoxes[hf[args__], fmt_] :=
Block[{Times}, MakeBoxes[HoldForm[args], fmt]] /.
RowBox[{"(", RowBox[{n_, "*", FractionBox["1", d_]}], ")"}] :> FractionBox[n, d]
Now using hf in place of HoldForm:
hf[1/2*3/4] // TeXForm
\frac{1}{2}*\frac{3}{4}
Formatted:
$\frac{1}{2}*\frac{3}{4}$
