Two restrictions in the theories that include $G(t)$ and $c(t)$ varying with time

Much work has been done taking into account the possibility that the gravitational {\it constant} $G$ may vary with cosmological time $t$ (or with the cosmological scale factor $a(t)$). The same may be said about the speed of light $c$. We present here two important remarks on these subject. These remarks include $G(t)$ and $c(t)$ varying with time with the restriction $8 \pi G / c^4 = \hbox{constant}$