Evaluate signal power between two amplifiers
How can I evaluate the SNR in such a case?

The signal power is \$V_{SIG}^2/R\$
The noise power is \$V_{NOISE}^2/R\$
So, when you take the ratio (aka signal to noise ratio or SNR) the resistance \$R\$ cancels and you are left with: -
$$SNR = \dfrac{V_{SIG}^2}{V_{NOISE}^2}$$