Hello kenislav
Can you tell me what kind of guitar you have? Does it have its own built-in preamp like an acoustic/electric guitar or is it just an electric?
Normally, you can connect the microphone for the violin into the mic input and connect the guitar to the Instrument input on the right side of the MixingLink. Then, connect the TO AMP output of the MixingLink to Input 1 of the H90. Then, you’ll be able to use Outputs 1&2 of the H90 for stereo.