Consider the following matrix function
$$
f(t) = \cos(\omega_1t) A_1 + \cos(\omega_2t) A_2, \quad t\ge 0,
$$
where $A_1$, $A_2$ are real square matrices and $\omega_1$, $\omega_2$ positive numbers.

Now, consider the following series of nested integrals
$$
S=\sum_{n=1}^\infty \int_0^{t} f(t_1)\left(\int_0^{t_1} f(t_2)\cdots \left(\int_0^{t_{n-1}} f(t_n)\, \mathrm{d}t_n\right)\cdots\mathrm{d}t_2\right)  \mathrm{d}t_1.
$$

If $\omega_1=\omega_2=\bar \omega$ then the above series converges to 
$$
 \exp\left({(A_1+A_2) \int_0^t \cos(\bar \omega \tau )\, \mathrm{d}\tau \,}\right)-I,
$$
so that, in this case, a (rather loose) upper bound to the norm of $S$ is
$$
\|S\|\le \exp\left({\frac{2\alpha}{\bar \omega}}\right)-1,
$$
where $\alpha:=\max\{\|A_1\|,\|A_2\|\}$.

> **My question.** With reference to the case $\omega_1\ne \omega_2$ does the following upper bound hold true
$$
\|S\|\le \exp\left({\frac{2\alpha}{\bar \omega}}\right)-1
$$
where $\bar \omega:=\min\{\omega_1,\omega_2\}$? [If this is not true, is it possible to find an upper bound on $\|S\|$ depending on $\bar \omega$?]